FEATURE: Let users select flair (#13587)

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.

app/serializers/current_user_serializer.rb

@@ -48,6 +48,7 @@ class CurrentUserSerializer < BasicUserSerializer
              :previous_visit_at,
              :seen_notification_id,
              :primary_group_id,
+             :flair_group_id,
              :can_create_topic,
              :can_create_group,
              :link_posting_access,