R1:3fedc8c299f7

Allow any EditEngine comment form to be pinned

Summary:
Fixes T12049. This expands "Haunted" comment panels to EditEngine, and by extension to all EditEngine applications.

Eventual goal is to remove custom commenting code in Differential and replace it with EditEngine code.

Changes from current "haunt" mode:

- This only has one mode ("pinned"), not two ("pinned", "pinned with preview"). There's an inline preview and scroll behavior is a little better.
- Now has a UI action button.

Slightly tricky…