[RFE] Site admins should be able to specify registration rules

[EmilyDirsh]

There are many valid use cases for site admins to want to restrict registrations to very specific types of email addresses or even specifc services. For instance, a site admin might want to only allow registrations from specific domains, or disallow + addresses, or only support a specific openid provider (cough fedora).

We should provide a way to specify registration restrictions as configuration options.

May be useful:
https://github.com/plataformatec/devise/wiki/How-to:-Use-a-custom-email-validator-with-Devise