Adding new invitation process for admins when they create new users.

Please review UserController.php at line #118 for the private function sendWelcomeMail($new_user) as I'm not sure if this is the most suitable location for this.

Also note that for add user view page I made substantial changes to hide all non-relevant input fields for the add user view eg. groups, permissions, etc.. have been hidden until after the user is created and the admin is redirected to the user edit page. I also added a new select to determine if the welcome email is to be sent to the new user or not. I decided to leave the "ban user" select in as that might come in handy to proactively ban a user that hasn't yet registered.

I might revisit this process again and give the admin the ability to assign group(s) and permission(s) to a new user all in one step.