R1:23bfe0c0f694

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…