Ability to completely disable the normal login and registration

[sudhirb]

We have been using this plugin to allow access to all users within our domain. We have restricted access via this to our domain users only. In addition we have set the Self-registration to Account activation by email. This enables all the users of our domain to happily login to redmine using their existing Google Apps login itself without any admin intervention. The only down side is that other users can also register through the normal process (not via this plugin/Google Apps login) and get an active login on the system. There have also been instances within our organization where in our users clicked on the register button and registered rather than using the Sign In With Google