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 + cancel.

• Edited comments; saved and canceled them. Used undo for changed state.
• Replied to comments; yada yada as above.
• Deleted comments.
• Did various modal trickery where I clicked "Reply" on something else with a

dialog already up, this very mildly glitches but I think it's not a big issue.

Reviewers: vrana, btrahan, Makinde, nh

Reviewed By: vrana

CC: aran, epriestley

Maniphest Tasks: T431

Differential Revision: https://secure.phabricator.com/D1716