Fix a self-XSS hole in Diffusion

Summary:
Via HackerOne. We aren't correctly escaping the date, so a user can XSS themselves by setting their date format creatively.

This construction is very unusual and I don't think we do anything similar elsewhere, so I can't come up with a systematic change which would prevent this in the general case.

Test Plan: Set date format to tag junk, got self-XSS before patch and proper escaping after the patch.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D12117