Provide live thumbs up/down icon on submit assessment button w/ tooltip.

This is intended to help resolve the problem of mentors accidentally
rejecting submissions they intend to accept and vice versa, as mentioned in

I didn't add unit tests to submission-detail.js right now since the
code is written in a way that it's ok if it throws exceptions (the
functionality isn't critical to the functioning of the site), and it's
also kind of hard to test regardless.
Merge pull request #81 from mozilla/live-submit-feedback

Provide live thumbs up/down icon on submit assessment button w/ tooltip.