R1:282d6e5ffa7b

Unify Maniphest + Differential comment styles

Summary:
I want to add comments to commits, and they should obviously share code with the
nearly-identical comments in Maniphest and Differential. Unify code/style as
much as possible.

This program made possible by a generous grant from D1513.

Test Plan:
- Looked at a bunch of different Differential and Maniphest comments; they
appeared to render identically to how they looked before.
- Tested some edge cases like anchors and "show details" on…