Followup to c03f83bbea.
The `flair_group_id` parameter is now required to show the flair, and this serializer was missing that detail.
This also fixes a typo in the `include_flair_group_name?` method.
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.
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
