Propagate the "ContextObject" to Remarkup rendering in timelines

Summary:
Ref T13602. Currently, timeline comment rendering does not (by default) propagate the context object to the rendering layer.

This means that @mentions of users who can't see the object aren't rendered properly (currently: they show up as blue, but should show up as grey).

Pass the context down the stack and into the remarkup engine.

Test Plan:

Maniphest Tasks: T13602

Differential Revision: https://secure.phabricator.com/D21548