Adds a new comment sort named "relevance" (may change the name) and
makes it the default. This sort takes into account the tags that have
been applied to comments and sorts them differently based on them. This
will certainly require a lot more work and adjustments, but for now it
will help push down noise, offtopic, and joke comments.
In the future, this can probably also be used to apply some more
interesting sorting effects, like potentially placing new comments
temporarily near the top of the comment section to help them get a bit
of initial exposure in large threads.