R1:2aac3156f791

Restructure Hovercards to support more context information

Summary:
Ref T13602. Currently, Hovercards are functions only of the object they represent (and the viewer, etc).

Recent changes to how users who can't see an object are rendered motivate making them a function of both the object they represent //and// the context in which they are being viewed. In particular, this enables a hovecard for a user to explain "This user can't see the thing you're lookign at right now.", so visual "exiled" markers can…