Reworked Actor: the only thing we know about an Actor is their URL.
Removed tests for Followers collections because it's hard to test
without subclassing the view. I may put it back in later.
The identifying URL is the only information we can be sure we have.
ThingUser's "name" field is modified similarly.
get_or_make() method is added to Actor.
ThingUser's as_activity() method is renamed to serialize() for consistency.