R1:21f0aba701b3

Use an inline dialog element for inline comments in Differential

Summary:
The current approach of using a modal overlay dialog to create/edit inline
comments is pretty silly. Use an inline textarea instead.

This element isn't perfect and we have some mild modalness issues, but I think
it's better than the silly thing we've got going on right now. We can keep
poking it as people break it.

Test Plan:
- Created comments; submitted and undid them in empty and nonempty states.
Used undo for nonempty states…