Add docs on group visibility

Accompanying https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3323

See merge request !3338

GitLab intro docs

Related to https://gitlab.com/gitlab-org/marketing_monthly_release/issues/1

---

Need refactor:

- Create a new project
- Create a new group
- Create a new issue
- Assign labels to issues
- Use milestones as an overview of your project's tracker
- Fork a project and contribute to it
- Create a new merge request
- Automatically close issues from merge requests (include GitLab.com pattern)
- GitLab CI quick start guide (make it easier to follow)

Moved to https://gitlab.com/gitlab-org/gitlab-ce/issues/8068

See merge request !3225

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CI: Add 'triggers' keyword to 'only' and 'except' lists to allow control over when triggers cause builds to run

Currently, the `only` and `except` keywords in `.gitlab-ci.yml` only accept ref names or the special `branches` and `tags` keywords. However, these are primarily useful when controlling how repository activity affects the creation of builds. In my case, instead of building on every commit, I'd like to use the following logic:

- If the repository is tagged, do a build.
- Any other normal commits should not cause a build.
- If a build is triggered via the API, always create one for the specified ref.

From what I can tell, this isn't possible via the existing YAML syntax. In this MR, I introduce a new keyword `triggers` that goes along with `branches` and `tags`. I can implement the logic above using the following job configuration:

```yaml
only:
  - tags
  - triggers
```

I updated the tests and documentation to reflect this and everything seems to pass.

See merge request !3230

Check if index exists before adding it

I got an error when updating GDK because it already exists.

[ci skip]

See merge request !3335

* I got an error when updating GDK because it already exists.

Add group visibility level

Supersedes https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3051

Closes #12658

See merge request !3323

# Conflicts:
#	app/models/issue.rb
#	app/views/projects/_home_panel.html.haml
#	app/views/shared/projects/_project.html.haml
#	db/schema.rb
#	spec/models/project_spec.rb

change the css class has_tooltip to has-tooltip universally

closes #14432 

See merge request !3321

Fix Indentation lints in CSS.

As discussed in #14299.

cc: @jschatz1 

See merge request !3312

Remove duplicated ago

Closes #14460 

See merge request !3332

Changing project icon

Closes #14196

See merge request !3268

Follow the CSS Style Guide rules for using shorthand where possible.

Fixes violations of the Shorthand rule. All of these were done manually and I reviewed each change to verify, so they should all be right.

Discussed in #14299.

cc: @jschatz1 

See merge request !3313

Soft delete issuables

Fixes #2489 

What still needs to happen: research on the indexes, the gem suggests a [lot of changes](https://github.com/rubysherpas/paranoia#about-indexes) though this is probably a good idea to discuss and I'm unsure on the impact of an omnibus upgrade as I suspect creating about 10 new indexes has a large impact on the downtime.

TODO: 
- [x] Also group owners can ***soft*** delete
- [x] Button should be hidden

See merge request !2982

Prevent tokens in the import URL to be showed by the UI

Quick fix to prevent the import URL to show a user's token.

See merge request !3331

Remove console.log call



See merge request !3322

Improve performance of viewing individual issues

This MR does two things:

1. `Issue#related_branches` no longer performs Git operations that aren't needed
2. The output of `Repository#exists?` is now cached and flushed properly

Combined these two changes should further cut down the amount of Git operations performed when viewing individual issues (and possibly other pages).

See merge request !3296

Ability to move issue to another project

Tasks:

- [x] Create scaffold of service that will move issue to another project.
- [x] Close old issue, add system note about moving issue to a new project.
- [x] Create a new issue, add system note about issue being moved from old project.
- [x] Check if issue can be moved to another project before executing service
- [x] Check permissions when moving an issue (`:admin_issue` ability)
- [x] Display select box for a new project when editing an issue
- [x] Show only projects that issue can be moved into in that select box
- [x] Add project select handler, helper and some permission filters to it
- [x] Preserve as much information as possible, including author
- [x] Prepare mechanisms that unfolds local references in issue description
- [x] Rewrite issue description with references unfolding and add some specs for it
- [x] Rewrite all system notes and comments attached to issue that is being moved
- [x] Update `Label` so that is was able to create cross reference labels (separate MR)
- [x] Add notifications about moving issue to another project
- [x] Display confirmation alert/message when issue move has been requested
- [x] Make it possible to undo selecting project where issue will be moved to
- [x] Add column to issue, that will indicate if it has been moved to another project
- [x] Do not allow to move issue that has been already moved
- [x] Write top-to-bottom feature spec in RSpec instead of Spinach

UI:

![issue_move_ui](/uploads/b3c6b563362c1fded9082cc0f51e5a74/issue_move_ui.png)

![issue_move_tooltip](/uploads/2ab913b06f52df1cafde9abe89bd9cb8/issue_move_tooltip.png)

Closes #3024

See merge request !2831

Use gitlab-workhorse 0.7.1

A small extra defense against interrupted git raw responses. Nice to
have in 8.6 because we introduce /raw/ caching; it would be
unfortunate if incorrect (truncated) responses got cached.

See merge request !3330

Fix diff image view modes (2-up, swipe, onion skin) not working

Example test case: https://gitlab.com/mrtzcspr/test/commit/5d983226b1308d98ef3d8919b978ad49f34496b8

Closes #13856

See merge request !3316

Use the configured Kaminari "per page" default

Configured in [`config/initializers/kaminari.rb`](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/config/initializers/kaminari_config.rb).

See merge request !3320