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 have a path forward toward discovery.

Test Plan:

• This change isn't expected to affect any behavior.
• Viewed hovercards, moused over/out, resized windows, viewed standalone cards, viewed debug cards, saw no behavioral changes.

Maniphest Tasks: T13602

Differential Revision: https://secure.phabricator.com/D21553