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 #75 and #76.

A modal yes/no dialog as suggested in #76 isn't ideal because users will most likely habituate to always confirming the action, thereby [accidentally confirming it](http://alistapart.com/article/neveruseawarning) the one time they'd want to reject it. So instead, I added some JS that provides a live indication of whether the submission will be accepted or rejected based on what the mentor has filled out so far.

If not all the required rubric items are checked, a thumbs-down icon will appear on the "submit assessment" button; when the user hovers over the button, a tooltip will appear like so:

![screen shot 2013-06-27 at 1 57 23 pm](https://f.cloud.github.com/assets/124687/717766/3ea810a6-df53-11e2-8376-5b071780f2e3.png)

If all required items are checked, however, a thumbs-up icon will appear with the following hover tooltip:

![screen shot 2013-06-27 at 1 57 38 pm](https://f.cloud.github.com/assets/124687/717771/538763fa-df53-11e2-8a13-ba0ff70dd256.png)

If folks are still making the mistake this solution is intended to resolve, we should either implement the modal confirmation mentioned in #76 (which is easy to implement, yet inhumane) or the undo/revise functionality mentioned in #75 (which is hard to implement, yet humane).

**Implementation note:** 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.