Return user objectwith self.current_user

This cleans up the codebase by returning a user object for self.current_user instead of just the email. Almost every instance of the call was just instantiating a user object anyway, so this makes that boilerplate. It also allows the base template to automatically get user info and level for rendering.
I don't believe this is safe as tornado expects `self.current_user` to be a string, right? This can easily have unintended consequences