Make celerity be able to render full uris

Summary: ...and use 'em in the phame blog case.

Test Plan: viewed blog.phabricator.dev and it actually looked right!

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1373

Differential Revision: https://secure.phabricator.com/D3666