Remove sign in when authenticating already authorized apps.

I'm submitting this to start a discussion on whether it'd be worthwhile and to raise any security concerns that it might cause. In particular, given that there have been reports of session issues with the forums, I want to make 100% certain that there would be no such issues with Core. Additionally, it might be worth making sessions expire when someone tries to use that session id from a different IP address, though this will cause shortened session times for those with dynamic IP addresses, or that switch the location of their computer often.