Less intrusive system note

## What does this MR do?
This MR is for making system notes less intrusive by implementing proposed design
 
## Are there points in the code the reviewer needs to double check?
System notes are reusing in issues, commits and MR discussions. May need to double check to make sure that the design is not breaking anywhere

## Why was this MR needed?
This merge request solve issue #19797 

## Screenshots (if relevant)
**Before:**
![merge-request-before](/uploads/91953598087a613078d80333a43815a7/merge-request-before.png)
**After**
![2016-11-15_17.41.46](/uploads/c809ba53cc34ee83bb439ed741dc39f3/2016-11-15_17.41.46.gif)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?
#19797, #23129

See merge request !6755

Slash command for mattermost

Closes  #22540

## Does this MR meet the acceptance criteria?

- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)


See merge request !7438

Use `Gitlab.config.gitlab.host` over `'localhost'`

Use `Gitlab.config.gitlab.host` over `'localhost'`

This would fix long standing failures running tests on
my development machine, which set `Gitlab.config.gitlab.host`
to another host because it's not my local computer. Now I
finally cannot withstand it and decided to fix them once and
for all.

See merge request !7562

Improved Ruby code in autocomplete user search

## What does this MR do?

Improves the Ruby code for finding users in the assignee dropdown based on the Ruby review.

See merge request !7576

Improve consistency of empty todo list messages

- They all now end with punctuation
- Not everyone enjoys, or can even drink, coffee
- The "High five!" message had no context for why we were high-fiving
- "Henceforth" is one word

[ci skip]

See merge request !7572

add parsing support for incoming html email

## What does this MR do?
Fixes #18388 by adding support for parsing HTML email

## Are there points in the code the reviewer needs to double check?
The new class, Gitlab::Email::HTMLParser, which needs to translate the HTML content to text and also delete replies, as they are not necessarily in the correct format to be caught by EmailReplyParser. The solution I found that should work for any HTML-formatted email is to remove all `<table>` and `<blockquote>` tags. Actual `<table>` elements (to be interpreted by markdown) should already be encoded with e.g. `&lt;table&gt;` - the only failure mode is if there is an *actual* HTML table in the content itself, which we wouldn't be able to support easily anyways.

The gem `html2text` traverses the HTML tree and outputs text - and markdown in the case of HTML links or images.

See merge request !7397

Resolve "spec/lib/gitlab/backend/shell_spec.rb causes other specs to fail"

## What does this MR do?

It ensures we rely on `Gitlab::Shell`'s public API and not on its internal. Since `Gitlab::Shell` is caching the value of its token, the only way to get the correct token is to call `Gitlab::Shell.secret_token`, not to read the `Gitlab.config.gitlab_shell.secret_file` file!

Fixes #24599

See merge request !7557

Document how to prevent artifacts from passing to next stage

See merge request !7575

[ci skip]

Remove title from skipped status svg

Closes #24617

See merge request !7570

Add CI example for Play/Scala apps with deployment to Heroku

Replaces https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3204

See merge request !7574

[ci skip]

Add environment info to builds page

![Screen_Shot_2016-11-02_at_5.44.01_PM](/uploads/3443d9518997147d1e6f41830e3774ff/Screen_Shot_2016-11-02_at_5.44.01_PM.png)

Closes #23205

See merge request !7251

Pass correct tag target to post-receive hook when creating tag via UI

Fixes #24437

See merge request !7556

- They all now end with punctuation
- Not everyone enjoys, or can even drink, coffee
- The "High five!" message had no context for why we were high-fiving
- "Henceforth" is one word

[ci skip]

Fix migration missing ignored transaction method

Adds missing `disable_ddl_transaction!` to migration

See merge request !7569

Add JIRA remotelinks and prevent duplicated closing messages

implements #13232  
closes #20479   

See merge request !7413

Fix misaligned buttons on admin builds page

## What does this MR do?
Fix misaligned buttons on admin builds page

## Are there points in the code the reviewer needs to double check?
No

## Why was this MR needed?
Because buttons are misaligned.

## Screenshots (if relevant)
![fix-admin-ci-table](/uploads/49472f7a12f868556e1bf482c9f837c4/fix-admin-ci-table.gif)

## Does this MR meet the acceptance criteria?

- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?
Fixes #24371

See merge request !7424

We need to handle annotated tags that are created via GitLab UI.
Annotated tags have their own SHA. We have to pass this SHA to
post-receive hook to mirror what happens when someone creates
an annotated tag in their local repository and pushes it via
command line.
In order to obtain tag SHA we first have to create it. This is
a bit confusing because we create the tag before executing
pre-hooks, but there is no way to create a tag outside the
repository. If pre-hooks fail we have to clean up after ourselves.

Correct curl examples for archive and unarchive project API

[ci skip]

See merge request !7559

Cycle Analytics: Events per stage

Adds list of events to each stage:

- Issue: list of issues created in the last XX days, that have been labeled or added to a milestone.
- Plan: list of commits that reference for the fist time an issue from the last stage.
- Code: list of MR created in this stage
- Test: List of unique builds triggered by the commits.
- Review: List of MR merged
- Staging: List of deployed builds
- Production: list of issues with the time from idea to production

Fixes #23449 

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !6859

More aggressively preload on merge request and issue index pages

See merge request !7564

Remove race condition while deleting groups

## What does this MR do?

The intended flow during a group deletion is:

```
  Soft-delete group (sync) -> Delete group projects (async) -> Hard-delete group (async)
```

The soft-delete was run in a transaction, which was committed only after
the async job (for hard-deletion) was kicked off. There was a race
condition here - the soft-delete transaction could complete _after_ the
hard delete completed, leaving a soft-deleted record in the database.

This MR removes this race condition. There is no need to run the
soft-delete in a transaction. The soft-delete completes before the async
job is kicked off.

This MR also adds a migration to delete all existing (soft-deleted) groups left in an inconsistent state
due to this bug.

- Closes #23223 
- EE merge request: gitlab-org/gitlab-ee!886

See merge request !7528