Remove trailing comma
Use facebook_id from facebook api instead of $_POST

Don’t use the facebook_id from $_POST because a user might have changed
that value in the html form.
Don't allow 0 as facebook_id

The Facebook api returns 0 if the user is not connected to Facebook.
Extract logging and session handling from `Session::login` into `Session::loginUser`

So we can reuse it when someone logs in with Facebook
Refactor login page

- use `Session::loginUser` to log in user (includes sending Sift Sience
login events)
- only show modal when user tries to connect Facebook account
- remove php notices
Merge pull request #53 from jonasdt/facebook

Refactor login page and fix facebook login bugs