Duplicate email addresses for users

[niconoe]

Discovered by @damianooldoni when reviewing #179:

Multiple users can have the same email address. This is not great since the email address can be used to reset passwords (then, multiple unrelated users will receive the option to reset their password).

We should make sure email addresses are unique in the database, whatever the route the user take (registration of a new user, editing an existing profile, manual user addition in the admin site, ...)

[niconoe]

@damianooldoni: it's now not possible anymore to have duplicate email addresses, can you check in the dev environment? By the way, I had to delete your d account since it was conflicting with the new constraint. You can recreate one, using another email address :)