Swap Source Sans Bold for Semibold

Summary: Testing this out, feels much less heavy and more readable for me, but open to other feedback. I've also corrected/normalize the header sizes and heights.

Test Plan:
Copied some real Phriction documents to my local install, read them all. Checked header sizes and make sure px are used for more absolute control over sizes.

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Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7963

Differential Revision: https://secure.phabricator.com/D12587