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:
{F5475505}
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