Include Source Sans on-demand with Celerity

Summary:
Unwinds the mess I made in D8422 / D8430:

  - Remove `'fonts'`, since individual fonts can be included via Celerity now.
  - Include Source Sans from the local source when a document uses it as a fontkit.

Test Plan: Browsed Diviner, saw Source Sans.

Reviewers: chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D8431