Summary:
Technically we didn't have it in the first place, but should. Also
add in X-Frame-Options for double-plus-good.
Test Plan:
Created a page with Phabricator in an <iframe />, got busted out
of it. Added in the X-Frame-Options, got an empty iframe.
Differential Revision: 38
Reviewed By: tomo
Reviewers: mroch, tomo
0b1450c5f9