Cherry-picked d2528b26b6
Conflicts:
- `app/serializers/initial_state_serializer.rb`:
Upstream changed stuff, we had extra attributes.
Applied upstream changes while keeping our extra attributes.
- `app/serializers/rest/instance_serializer.rb`:
Upstream actually moved that to `app/serializers/rest/v1/instance_serializer.rb`,
so updated that file by keeping our extra attributes, and took upstream's
version of `app/serializers/rest/instance_serializer.rb`.
- `spec/views/about/show.html.haml_spec.rb`:
Took upstream's version.
* Move ApiController to Api/BaseController
* API controllers inherit from Api::BaseController
* Add coverage for various error cases in api/base controller
