Email notification to an admin whenever a user signs up

[furilo]

We could have it as an option on the site settings.

[bertocq]

I think I can take this one. Any thoughs about spec details? Or better for me to roll a PR and complete them bit by bit there?

[ferblape]

As you can might see we use a pattern named form objects to abstract the input data of an action, you can read more here. It's enough to add a test to the mailer and a test to the form object to be totally covered.

Thanks!