Display name should not allow an e-mail address

[mvl22]

There are a small number of users with Display Name as an e-mail address.

This needs to be prevented in future.

Because e-mails get issued in the form "Display name" <notifications@example.com>, this means the From: address has two e-mails within it, which SpamAssassin and other systems treats a spam indicator, causing high spam likelihood scores of 8 or so.

This just needs a regexp disallowing any use of @ (as even @username is likely to turn into an e-mail when quotes added).

[mvl22]

All existing users with @ in their username have been migrated to using at.