Provide user option for disabling diffusion symbol cross-references

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