Re-work authentication, user creation and redirect after login.

[richardkmichael]

UsersController#create redirects to the created user, but is also used as auth.

This means the user cannot visit a URL, be prompted for auth, then redirected to the original requested URL (because the login process will redirect them to their user page - as above).

We need a LoginController which will auth (creating an account if necessary) and redirect to the original requested URL. UsersController#create should be reserved for completion of a user profile.

No where in the application will a user be able to create another user from scratch - ex. no "New User" button exists. This means we do not need a UsersController#create action, but only #update - when a user completes their profile. LoginController#create will create the user account, if required.