User flair was given by user's primary group. This PR separates the
two, adds a new field to the user model for flair group ID and users
can select their flair from user preferences now.
ListableTopicSerializer includes many attributes which we are not using, and is likely to cause N+1s when not used in conjunction with TopicQuery.
Using the BasicTopicSerializer means that no other tables are required.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
Added "Top Links" list
Added "Most Liked By" list
Added "Bookmark count" stat
UX: Use fa heart icon instead of "like" text in stats
Change the order of the user stats
