User has to confirm deletion of issuables

Closes #14474

See merge request !3341

[ci skip]

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

[ci skip]

[ci skip]

[ci skip]

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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