Fix symbol URI generation to include default branch name for relevant repositories

Summary: We need to build a request in order to pick up an appropriate default branch name, instead of using the raw static generator.

Test Plan: Clicked a symbol link, got /master/path/blahblah

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1982