Remove docs sprite, replace with FontAwesome

Summary: Removes the docs sprite in Conpherence with FontAwesome, adds additional icons. Unsure what happens if someone customized this config option.

Test Plan: Added images and files to a Conpherence, saw new icons.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11028