Sort ghost children correctly in Phriction

Summary:
Currently, if some page /x/ has a grandchild at /x/y/z/ but /x/y/ does not exist, we insert a ghost entry for it in the "Document Hierarchy". However, we don't sort the hierarchy properly after inserting ghosts, so they always appear at the bottom and in an unuseful order.

Instead, sort children again after any ghost insertions.

Test Plan: Created /x/a/, /y/a/, /z/a/, verified ghosts sorted incorrectly before this patch and correctly afterward.

Reviewers: btrahan, vrana

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1750

Differential Revision: https://secure.phabricator.com/D3418