Support WOFF files in Celerity and add Source Sans Pro

Summary:

• Allow Celerity to map and serve WOFF files.
• Add Source Sans Pro, Source Sans Pro Bold, and the corresponding LICENSE.
• Add a font-source-sans-pro resource for the font.

Test Plan:

• Changed body font-face to 'Source Sans Pro'.
• Added require_celerity_resource('font-source-sans-pro') in StandardPageView.

Works in Firefox/Chrome/Safari, at least:

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Reviewers: btrahan, chad

Reviewed By: chad

CC: chad, aran

Differential Revision: https://secure.phabricator.com/D8430