Merge branch 'mrchrisw/remove-whitelist' into 'master'

Update user whitelist reject message

## What does this MR do?

Makes email whitelist rejection message more generic. Stops printing all `allowed_domains`

## Why was this MR needed?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/23090

See merge request !6731

app/models/user.rb

@@ -902,7 +902,7 @@ class User < ActiveRecord::Base
       if domain_matches?(allowed_domains, self.email)
         valid = true
       else
-        error = "is not whitelisted. Email domains valid for registration are: #{allowed_domains.join(', ')}"
+        error = "domain is not authorized for sign-up"
         valid = false
       end
     end