Render user hovercards with context information about their ability to see the context object

Summary:
Ref T13602. When rendering a user hovercard, pass the object on which the reference appears. If the user can't see the object, make it clear on the hovecard.

Restyle the "nopermission" markup in mentions to make it more obvious what the style means: instead of grey text, use red with an explicit icon.

Test Plan:

Maniphest Tasks: T13602

Differential Revision: https://secure.phabricator.com/D21554