[Redesign] Move basefont to Lato, remove Source Sans Pro

Summary: Working towards a more unified look and feel. This brings in Lato as a complete base font over Helvetica Neue, as well as removing Source Sans Pro from DocumentView and Conpherence. Design-wise Lato provides the nice readability at larger font sizes that Source Sans Pro did, with the ability to scale down to tables and UI widgets with ease. This gives us one font instead of two, and now Object descriptions and Timeline posts all can benefit from a consistent, readable font.

Test Plan:
Test main UI, smaller elements like tables, menus, DocumentViews, Previews, Conpherence.

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Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D13276