Show child documents on Phriction pages

Summary: Pull the next couple levels of hierarchy and render them at the bottom
of the page. This might need some tweaking but it seems OK at first glance.
Test Plan:
https://secure.phabricator.com/file/info/PHID-FILE-ef0af5d4dc6dceaeb2e3/

Also reduced limit to 1 and verified the "more" behavior worked properly.

Reviewed By: hsb
Reviewers: hsb, codeblock, jungejason, tuomaspelkonen, aran
CC: aran, hsb
Differential Revision: 676