Render object monograms and embedding references more gracefully when building a Remarkup table of contents

Summary:
Fixes T8845. Ref T13102. See PHI467. Currently, object monograms like L1 which appear in Remarkup headers render incorrectly (with an internal placeholder "x") in the table of contents:

Instead, render them down to just, e.g., L1 in plain text.

For {P123} I just rendered it to {P123} since it's not really clear to me what users intend. This could be adjusted if there's some reasonable thing that someone is trying to do with this.

Test Plan: Wrote a Phriction document with several object references (like L1 and {P123}) in headers. After patch, saw "x"-free, sensible-looking header names in the table of contents.

Maniphest Tasks: T13102, T8845

Differential Revision: https://secure.phabricator.com/D19234