Merge branch 'upgrade-rubocop' into 'master'

Upgrade Rubocop from 0.35.1 to 0.38.0

See [Rubocop's Releases page](https://github.com/bbatsov/rubocop/releases) for more info on what changed.

Descriptions/StyleGuides removed as discussed in #14233.

Changes:
- Enable DisplayCopNames for lint output.
- Default behavior for `Alias` changed, set to enforce `prefer_alias_method`.
- Enabling Rails cops changed to new syntax.
- Remove StyleGuides and move Descriptions to comments.
- Add missing cops.
- Add TODOs for cops that should be enabled in the future.
- Set TargetRubyVersion to 2.1.

See merge request !3197