V14: Deny login screen access

[Zeegaan]

Notes

• In some scenarios, you could manually navigate to the login screen (/umbraco/login), where you were not really supposed to be able to do that:

1. When you are already logged in, you should not be able to see the login screen
2. When you are resetting a password, and already logged in
3. When you are invited, but already logged in

From a server perspective, we needed a way to tell the client about the logged in status of the user, thus we have added a property to the model that get's send with, with whether the user is logged in or not.

How to test

• Log in the backoffice
• Manually navigate to /umbraco/login, if you set a breakpoint, you should be able to see the UserIsAlreadyLoggedIn property being set to true.