Serve all fonts locally

Special fonts like FontAwesome and Glyphicons were already served locally,
but Source Sans Pro was served from Google Fonts service.

To improve privacy, the fonts have been imported in the repository,
to allow Workspaces instances to serve the font locally.

Font license: SIL Open Font License 1.1
Copyright 2010, 2012 Adobe Systems Incorporated

Ref T1881

Test Plan: Tested on obsidian51. with inspect elements

Differential Revision: https://devcentral.nasqueron.org/D4222