Facebook auth support.

This change in logic means we'll be identifying a user two times on the "sign in" page rather than once, but thumbing through the segment.io docs there doesn't seem to be any problem with doing this.  If there is, we could figure out something else—I didn't want to break up this flow too much in the event that the identifications are being used in other ways throughout pages that embed this segment.io widget, but I wanted to move away from the manual URL parsing