Add an option to not generate passwords for users

[LloydW93]

Currently, new users are created credentials for MyRadioDefaultAuthenticator. It should be possible to specify the credential provider for new users, to which this is delegated.

I'm thinking a configurable called newUserAuthenticator, and an extension to the MyRadioAuthenticator Interface called something like provisionAccount, which takes either a memberid or email, and responds with something that can be included in the welcome email. For example, MyRadioDefaultAuthenticator could return "Your new password is: Radio43Meow", and MyRadioLdapAuthenticator could respond "Use your IT Services password to log in."