Allow subscribing to labels

This implement #12743 and supersedes !2799 (thanks @timothyandrew!).

- Subscribe (and unsubscribe) for labels
- When an issue/merge request is created, notify all subscribers of all its labels
- When an issue/merge request is edited, notify all subscribers of all its newly-added labels

## Done

- [x] Verify that a user is signed in to subscribe/unsubscribe from a label.
- [x] Merge conflicts
- [x] Integration tests
- [x] `issuable.subscribed?` should check subscribers and participants
- [x] When an issuable is relabeled, notify subscribers of the *newly added* labels only
- [x] Screenshots:

### Labels page

![Screen_Shot_2016-03-07_at_19.05.44](/uploads/3e69064e9e1a06ee2e27c778776c1407/Screen_Shot_2016-03-07_at_19.05.44.png)

### HTML email

![Screen_Shot_2016-03-07_at_19.07.36](/uploads/e484e06366a738385f1f6d71b52eecf7/Screen_Shot_2016-03-07_at_19.07.36.png)

### Plain text email

![Screen_Shot_2016-03-07_at_19.07.50](/uploads/dc05f710a8f7ab5eae9f72aa2110e741/Screen_Shot_2016-03-07_at_19.07.50.png)

PS: I've set the milestone to 8.6 since it's getting late for 8.5...

See merge request !3115

1. Make the "subscribed" text in Issuable sidebar reflect the labels
   subscription status

2. Current user mut be logged-in to toggle issue/MR/label subscription

[ci skip]

CSS style guide

Working towards what was discussed in #13552, this adds the [SCSS Linter gem](https://github.com/brigade/scss-lint) for style guide conformity in CI.

TODO:
- [x] Agree on and write SCSS Style Guide Documentation.
- [x] Document the `scss-lint` config file.
- [x] Figure out how best to run this in CI, right now it's taking longer than I would think it should.
- [x] Use CSSComb for auto-correction (Maybe just include a CSSComb config file and have developers run the node package manually if they're interested in using it?).

My logic for not using CSSComb in the first place is that, AFAIK, we don't currently require Node/NPM, even in the dev environment. Maybe I'm wrong about that? `scss-lint` is a Ruby implementation of a similar concept, but it doesn't include autocorrect.

Is there a way we can run get GitLab CI to run `scss-lint` without having to put together the full GitLab application? Seeing as it's just static analysis, it seems like a waste of time/resources.

/cc @jschatz1 @rspeicher

See merge request !3069

Remove parentheses from if statement

Otherwise Rubocop will give a warning after we upgrade it.

As discussed in !3197.

[ci skip]

See merge request !3231

Otherwise Rubocop will give a warning.

As mentioned in !3197.

1. Allow subscribing (the current user) to a label

- Refactor the `Subscription` coffeescript class
  - The main change is that it accepts a container, and conducts all
    DOM queries within its scope. We need this because the labels
    page has multiple instances of `Subscription` on the same page.

2. Creating an issue or MR with labels notifies users subscribed to those labels

- Label `has_many` subscribers through subscriptions.

3. Adding a label to an issue or MR notifies users subscribed to those labels

- This only applies to subscribers of the label that has just been
  added, not all labels for the issue.

Ignore eager loading in Project.search UNION

This fixes issues such as filtering groups by names on pages such as
https://gitlab.com/dashboard/groups.

See merge request !3229

Set link title for todos. Closes #14042

See merge request !3161

See merge request !3218

Fix typo in Ruby CI test and deploy example

[skip ci]

See merge request !3201

Revert the avatar cropping feature

Reverts !2951 since the added 'mini_magick' dependency should be replaced by client-side croppping

See merge request !3223

Fix double borders around the CI status

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/14281
Introduced by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2760#note_4238291

See merge request !3224

The queries that are UNION'd together don't need any eager loading
(since we really only use the resulting SQL instead of having
ActiveRecord actually run the queries). By dropping any eager loaded
associations queries such as the following work instead of producing a
SQL error:

    Project.all.includes(:namespace).search('foo')

Fixes "iid of max iid" in Issuable sidebar for merged MR


Fixes #13928.

See merge request !3046

Ensure "new SSH key" email do not ends up as dead Sidekiq jobs

Related to #2235.

This is done by:
1. Delaying the notification sending after the SSH key is commited in DB
2. Gracefully exit the mailer method if the record cannot be found

/cc @dblessing @stanhu @DouweM 

See merge request !3163

bundle:audit job only on master

As discussed in !3215

See merge request !3227

Applications tab on profile settings

Closes #13855

See merge request !3031

This reverts commit 01160fc0, reversing
changes made to 4bff9daf.

Run 'git gc' every 10 pushes

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/14250

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/12620

Fixes https://gitlab.com/gitlab-org/gitlab-ee/issues/254

Associated gitlab-shell MR:
https://gitlab.com/gitlab-org/gitlab-shell/merge_requests/44

See merge request !3206

Windows CI support is full since Runner v1.0.0



See merge request !3219

[ci skip]

Related to #2235.

This is done by:
1. Delaying the notification sending after the SSH key is commited in DB
2. Gracefully exit the mailer method if the record cannot be found

This solves the problem with caching the nil value with instance variable.
Without this the every time we ask for external_issue_tracker we built AR and potentially do SQL query

Fix bug with JS error from dropdowns in filter area

Fixes:

![consoleerror](/uploads/0fa19a943050baac855c9e6f896ea941/consoleerror.png)

This error is not a logged issue yet.

Error when turbolinks fetches a new page, `setup()` was not run. 

Signed-off-by: Jacob Schatz jschatz@gitlab.com

cc @iamphill @alfredo1 

See merge request !3216

Redesign project tabs on group page
Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>


Final step to fix #13480 

See merge request !3209

Be more intelligent about sanitizing links with unsafe protocols

This prevents false matches on relative links like
`[database](database.md)`.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/14220

See merge request !3210