Move User registration out of the AccountController

[lcharette]

User creation logic should be factored out of the AccountController, especially the unique user name and email logic.

This will make it easier to create a create-user Bakery command since all verifications prior to the user creation won't be duplicated, but centralized. When writing tests, Factories can also produce false positive since they can generate the same name/email twice.

Note that this is currently an issue since there's major code duplication between AccountController and CreateAdminUser

The User model could be a nice place to do this.

[lcharette]

This would probably makes things easier for #623.

[lcharette]

Implemented in bd30fa8 for V4.2

Absolutely not perfect, but better than before.