Send email if registration application was denied

[Nutomic]

The admin form for denying a user's application includes a "reason" field. However it seems that this field is not shown to the user at all. In particular, if the user provided an email during registration then the deny reason should be sent by mail, letting the user know that he can register on a different instance instead (eg with a joinlemmy link).

Note that lemmy-ui also doesnt show the deny reason when trying to login.