For Facebook and Twitter, leverage AS to find the user url and image.

In the case of Twitter, we can move some special case handling to prefer https
and to grab larger pictures out of bridgy and into activitystreams.

Aesthetically, it feels a little weird to half-construct the Twitter and FacebookPage
objects before calling as_source.user_to_actor and then filling in the rest of the 
fields... is there a better way to handle that?