Sketch key APIs and designs for anticensorship features

I'm going to try to get this done early to assure that we see any snags for %Arti 1.1.0: Anticensorship ready ahead of time.

changed milestone to %Arti 1.1.0: Anticensorship ready

assigned to @nickm

added Doing label

See !686 (merged) for first steps.

See !704 (closed) for API discussion.

mentioned in merge request !741 (merged)

mentioned in commit 1b53544e

closed with merge request !741 (merged)