CLA API and OAuth2 Provider Spike

:construction: This explores what it would look like to implement a basic CLA API using OAuth2 for authentication (via [Doorkeeper](https://github.com/applicake/doorkeeper)). In order for this to be mergeable the following should happen.

- [ ] Add tests for the API endpoint
- [ ] Override Doorkeeper controllers so applications are scoped only to the current user
- [ ] Override Doorkeeper views so they fit in with the rest of the application.