Move inline replies to new code and remove DifferentialInlineEditor

Summary:
Ref T12616. This moves "reply" to the new stuff and deletes DifferentialInlineEditor, which no longer does anything.

(This breaks some keyboard shortcuts, but I'll rebase D17859 shortly.)

Test Plan: Replied to inlines; things seemed to work properly.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12616

Differential Revision: https://secure.phabricator.com/D17894