Modified FB login, now add info to profile

There were issues if a user decided to log in with facebook after
previously registering with email, whereas now that information is
checked and if it doesn't exist it is added to the user's existing
profile.

This will pave the way for importing user friend lists and being able to
post stuff directly to facebook, after which I'll move onto the next set
of social logins.

I also changed a callback to remove null values in the friends requests
of the user document, I was using an old method that didn't ensure that
the null value was removed. This should be resolved now.