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

# Conflicts:
#	db/schema.rb

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