Summary: Some people don't like these, so they should be able to turn them off.
Test Plan:
Toggled the setting on and off; loaded a page in diffusion and differential
that should have symbol cross-references, and saw that they weren't linked
when I had the setting disabled. I also checked that the symbols are still
linked when the setting hasn't been touched.
Reviewers: epriestley, vrana
Reviewed By: epriestley
CC: aran, Korvin
Differential Revision: https://secure.phabricator.com/D3069
14e3e5ccfd