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.

db/migrate/20210625203049_add_flair_group_id_to_users.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddFlairGroupIdToUsers < ActiveRecord::Migration[6.1]
+  def change
+    add_column :users, :flair_group_id, :integer, null: true
+  end
+end