Turn off write guard when saving differential comment cache

Summary:
It is possible to view a comment that has no cache; when viewing such a comment
the request doesn't have a csrf token and there is no need for one, so we turn
off the write guard.

Test Plan:
loaded an old diff that had no cache, and the page loaded instead of throwing
an AphrontCSRFException.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: 858