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.

Previously we were doing all of kinds of code gymnastics and flash abuse
in order to work with a Doorkeeper controller but have it _appear_ at
the `/profile/applications` path. Fortunately we can just tell Rails to
use a different controller to handle that route, and we get the best of
both worlds.

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

The only major difference with the EE version is the change from a light and dark logo to only a header logo
The dark logo wasn't used anyway, so it seemed to make sense to me to rename the field to the actual function of it

Discussion:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2560#note_3742042

Also added API specs for that

Closes #11853

Currently any spam detected by Akismet by non-members via API will be logged
in a separate table in the admin page.

Closes #5612

This is the git default and will help to prevent false positive matches.

Closes #12706

Since GitHub doesn’t apply these constraints to theirs wiki slug
allowing characters like `,`, `:`, `*`, etc, we need to relax our
constraints or some wiki pages will not be available after they were
imported.

For an example the Devise project have a wiki page with the following
slug: “How To: Add sign_in, sign_out, and sign_up links to your layout
template”

Added also a `Gitlab::Ci::Build::Artifacts::Metadata::ParserError`
exception class.

This reverts nesting artifacts controller in builds module.