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
Point the Phabricator resource map back to webroot/, not webroot/rsrc/

Summary:
Ref T4222. Earlier, I adjusted the root from `webroot/` to `webroot/rsrc/`. However, this means that all the `/rsrc/x/y/z.jpg` fragments in CSS are no longer recognized as resource names.

Since we have like 9,000 things in CSS that do `url(/rsrc/xyz.jpg)` and I don't want to fix/test them all, so just make them work as-is. There's no real reason either setting is better than the other.

(Both URLs also work fine, but the parsed one will be better once we have real CDN support.)

Test Plan: Verified CSS gets managed resource URIs transformed into it.

Reviewers: btrahan, hach-que

Reviewed By: hach-que

CC: aran

Maniphest Tasks: T4222

Differential Revision: https://secure.phabricator.com/D7875