R1:c52e10d1ecf0

Respect external unmentionable PHIDs in Differential revision editor

Summary:
See PHI574. Ref T13120. When you `Ref Txx` or `Fixes Txxx`, we mark it "unmentionable" to prevent the task from generating both a reference and a mention.

If you add a reference to an object (like a commit hash) to a custom remarkup field, there's currently no real way to prevent it from generating a mention, except that you can explicitly mark the PHID as unmentionable on the Editor.

This isn't exactly a first-class feature…