Added a "Signed In" event to tracking.

We didn't previously have an event to track generic user sign ins. This pull request adds it.
I might be missing something in the login code, but it looks like a user already having a wpcom site in the app can add a self-hosted site and this value would be updated.  I'm unsure if that's the desired behavior but I'm guessing its not. Do we need a check to see if there is a default wpcom acct present?