When replying to a threaded inline, put the new inline in the right place in the thread

Summary:
Fixes T10563. If you have a thread like this:

> A
  > B
  > C

...and you reply to "B", we should put the new inline below "B".

We currently do when you reload the page, but the initial edit goes at the bottom always (below "C").

Test Plan: {F4963015}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10563

Differential Revision: https://secure.phabricator.com/D17938