This change updates the review-together extension, porting the JS side

to ES6, updating the Python code for Django 1.11/Python 3/Review Board

4.0 compatibility, and converting the CSS to use our new color

definitions.
The visual appearance isn't super polished (many of the colors are a

little bit clashy with TogetherJS's icons), but I haven't changed

anything there. Updating to the latest TogetherJS and polishing the UI

might make for a good student project at some point.

Installed and activated the extension. Verified that the settings form

and frontend UI worked as expected.