Change user creation form to allow Raven-only users

[thewilkybarkid]

When an administrators adds a user they have to add in a password, this isn't great for Raven-only users as they have to manually make one up. An option should be added to select whether the user should log in with Raven only or not (defaulted to true if the Raven override is enabled), which hides the password field if true.

The user_password() function should be used to generate a random one to avoid any potential security issues.