Show configurable level in the Document Hierarchy in Phriction.

Summary: Added phriction config option to specify the number of levels of hierarchy to display in the Document Hierarchy. Cleaning up my original pull request at https://github.com/facebook/phabricator/pull/338

Test Plan: After changing config option 'phriction.hierarchy-display-levels', document hierarchy will display that many levels.

Reviewers: epriestley, #blessed_reviewers

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8061