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

app/mailers/user_notifications.rb

@@ -37,6 +37,14 @@ class UserNotifications < ActionMailer::Base
                 new_user_tips: tips)
   end
 
+  def signup_after_reject(user, opts = {})
+    locale = user_locale(user)
+    build_email(user.email,
+                template: 'user_notifications.signup_after_reject',
+                locale: locale,
+                reject_reason: opts[:reject_reason])
+  end
+
   def suspicious_login(user, opts = {})
     ipinfo = DiscourseIpInfo.get(opts[:client_ip])
     location = ipinfo[:location]