Add support for content vs speaker rating

Should be 100% BC, allowing `rating` instead of the separate fields. If you submit `content_rating` then you must also submit `speaker_rating`.

Responses should also be BC.

**Note:** This PR has a new DB migration.

Exposed via the UI in joindin/joindin-web2#112
Updates to joind.in site to handle schema changes here: joindin/joind.in#798