Move Celerity gradually toward multiple source support

Summary: Ref T4222. This doesn't actually support multiple sources yet, but moves us closer by getting rid of some dead and exceedingly-singletoney code.

Test Plan: Browsed around, looked at Phame blogs.

Reviewers: btrahan, hach-que

Reviewed By: hach-que

CC: aran

Maniphest Tasks: T4222

Differential Revision: https://secure.phabricator.com/D7874