Fix/Login with Google

[Zjyslav]

There were still some problems with login and registration using Google. I think I fixed them. Now it works great for me.
What I did:

• I added some null checks and made sure claim values we work with are not null at any point.
• I placed a call to log out of Google claims at a better place. There was a call before, but it was so far down, that if you failed to log in, you wouldn't get logged out of Google.
• I changed LogInWithGoogleAsync to use email instead of username, because username is much more likely to change (both on our side and on Google's) than email. We can't have a situation in which I decide to drop my middle name from my Google name and can no longer login to TutorLizard.
• I did some refactoring, so that we have one method handling signing in based on LogInResult instead of the same logic duplicated. That way we can ensure user claims are consistent no matter what login method is used.
• Some minor changes to names and removal on unnecessary code fragments.