Make "Reply" on ghost inlines work correctly (or, at least, consistently)

Summary:
Ref T7447. Ref T7870. When you "reply" to a ghost inline, make it work properly.

This exact behavior is arguable. In particular, when you reply to a ghost inline, we could put the reply on the same diff as the original.

I suspect it aligns better with user exepectation to put the new inline on the current (visible) diff instead, and generally for inlines to flow forward through time and all of the ghosts to pretty much be older than all of the non-ghosts in most cases. We can see how it feels and adjust things if this turns out to not make sense.

Test Plan:

• Replied to ghost inlines, got new inlines on the proper display line.
• Replied to normal inlines, got normal behavior.
• Made some new inlines.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7870, T7447

Differential Revision: https://secure.phabricator.com/D12492