This change adds a new `ReviewRequestDetailsMixin` for common methods between `ReviewRequestResource` and `ReviewRequestDraftResource`. This currently contains the serializer for the `bugs_closed` field. While doing this, I noticed that the draft resource had a serializer for `status`, which is incorrect (there's no status field on the draft and the resource doesn't offer it, so it was just a dead method). This also modernizes the imports and class definitions in the mixins module. Testing Done: Ran unit tests.