Specialize rendering of self-URIs in the form "/X123"

Summary:
Depends on D20510. Ref T5378. When remarkup includes a hyperlink to the current install in the form "/X123" (which is common), load the corresponding object and specialize the rendering.

This doesn't cover everything (notably, no handling for Diffusion paths yet), but does cover a lot of the most common cases.

The "uri" form preserves the URI as written, but adds an icon, tag, and hovercard.

The "{uri}" form is more similar to {T123} and shows the object name.

Test Plan:

Reviewers: amckinley, joshuaspence

Reviewed By: joshuaspence

Subscribers: joshuaspence

Maniphest Tasks: T5378

Differential Revision: https://secure.phabricator.com/D20512