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.

* master: (177 commits)
  Add changelog
  Bump gitlab-shell version to 5.8.0 to fix Git for Windows 2.14
  Make contextual sidebar collapsible
  Fixed sidebar context header hover colors
  Use correct `Environment`-class within `Gitlab` namespace
  Remove gl.Activities from Commits page
  Move `let` calls inside the `describe` block using them
  Add `/assign me` alias support for assigning issuables to oneself
  GRPC::Unavailable (< GRPC::BadStatus) is wrapped in a CommandError
  Use `broken_storage` in the fs_shards_spec.
  Eager load project creators for project dashboards
  Memoize a user's personal projects count
  Remove redundant query from User#recent_push
  Improve checking if projects would be returned
  Change spelling of gitlab-shell
  Remove unused #tree-holder
  Add custom linter for inline JavaScript to haml_lint
  Rename user_can_admin? because it's more accurate
  Synchronous zanata community contribution translation
  Add Korean translation to i18n
  ...

This makes navigating through diff files quickly. Currently we just
toggle a list, which could be pretty big. This moves it into a dropdown
to make it much easier.

Also includes a filter bar to quickly search for certain
files/extensions.

Closes #29778

When you change the diff view (inline / side-by-side), we set a cookie based on
that new view. When you add a comment, we choose the style to use in the
response based on that cookie.

However, when you have just changed diff style, the request cookie will contain
the old value, so we should use the view param instead.