R1:94d0adb1406d

Add "Undo" for editing Differential inline comments

Summary:
When a user hits 'cancel' on a 'new', 'edit', or 'reply' operation, add a little
"Changes discarded. __Undo__" insert so they can get their change back. No undo
for delete since there's an explicit prompt. Once this lands we can make
'escape' work again to close dialogs.

This change started feeling really good when I was merging all the duplicate
code and making things more consistent, but by the time I started writing client
rendering it felt…