Adds Rubocop rule for line break after guard clause

Closes #18040

See merge request gitlab-org/gitlab-ce!15188

Adds a rubocop rule (with autocorrect) to ensure line break after guard clauses.

Implement /internal/post_receive unified endpoint for PostReceive tasks

See merge request !13916

The initializers including this were doing so at the top level, so every object
loaded after them had a `current_application_settings` method. However, if
someone had rack-attack enabled (which was loaded before these initializers), it
would try to load the API, and fail, because `Gitlab::CurrentSettings` didn't
have that method.

To fix this:

1. Don't include `Gitlab::CurrentSettings` at the top level. We do not need
   `Object.new.current_application_settings` to work.
2. Make `Gitlab::CurrentSettings` explicitly `extend self`, as we already use it
   like that in several places.
3. Change the initializers to use that new form.

And the link to the last page cannot be `page=0`.