Add skip_doorkeeper_for (and other refactors)

- Use `ActiveSupport::Concern` to refactor `Doorkeeper::Helpers::Filter`.
- Add `skip_doorkeeper_for`.
We'll be able to simplify this snippets with syntax with normal Ruby method parameters, described in https://github.com/doorkeeper-gem/doorkeeper/commit/1e3cebf1d88c572f4a8bfca224b6da2a4fcf6d55.