Remove unused forums-related cruft
Remove redundant URI.min.js fron lms/

A newer version was added to common/, so that is used instead. It is
also added to the set of JS files used in unit tests to allow forums
code in common/ to be tested.
Improve accessibility of forum vote buttons

This change involves substantial refactoring of the relevant code to
reduce duplication. It also makes the templates for the vote buttons
more consistent and cleaner.