FIX: Do not show invalid option in flair chooser (PR #13725)

Both of the commits in this PR are meant to fix the problem of invalid option being shown in the flair chooser. An invalid option can be shown if at some point it was a valid one - a group with a flair that was later changed by an admin and flair was removed. The other option an invalid option can be selected is if the user had a primary group when the migration ran and copied the same value to the flair_group_id column.

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