FEATURE: reason to reject user signup (#11700)

Feature for `Must Approve Users` setup. When a user is rejected, a staff member can optionally set a reason for audit purposes. In addition, feedback email can be sent to the user.

Meta: https://meta.discourse.org/t/account-rejection-email/103112/8

db/migrate/20210111025920_add_reject_reason_to_reviewables.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddRejectReasonToReviewables < ActiveRecord::Migration[6.0]
+  def change
+    add_column :reviewables, :reject_reason, :text
+  end
+end