New UI for the changes to talk rating

This new UI allows users to rate a talk on two criteria:

- The quality of the speaker (the presentation)
- The quality of the content

The new UI:

- Shows BC rating if no split rating has happened
- When rating, each "star" has a descriptor to promote consistency across users voting choices (rather than arbitrarily choosing what the numbers mean to them)
- Displays Overall/Content/Presentation ratings for the talk
- Hides comment rating breakdown until hover
- Hides speakers ratings completely
- Highlights speakers as speakers

Relies on joindin/joindin-api#67

You can see the changes in a short video, here: https://www.dropbox.com/s/jvbo86fyain5llo/Joind.in%20Ratings%20Improvements.mov