R1:981f3a9068bb

When marking up Phurl URLs for mail, use absolute URLs

Summary: Fixes T10625.

Test Plan: Faked this locallly and it looked OK, I'll check the mail in production. :3333

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10625

Differential Revision: https://secure.phabricator.com/D15497