FIX: Set user flair group to primary group (#13718)

This is a follow up to commit 87c1e98571
which introduced different fields for primary and flair groups. Before
that, primary group was used as a flair group too.

db/migrate/20210713092503_set_users_flair_group_id.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class SetUsersFlairGroupId < ActiveRecord::Migration[6.1]
+  def change
+    execute <<~SQL
+      UPDATE users
+      SET flair_group_id = primary_group_id
+      WHERE flair_group_id IS NULL
+    SQL
+  end
+end