Add setEpoch for PHUIHeaderView, use in all Documents

Summary: Sets a consistent last update time in the header of PHUIDocuments, Legalpad, Diviner, Phriction. I'm not set on the exact language, just that there is consistency, feel free to suggest changes.

Test Plan:
Test Legalpad, Diviner, Phriction.

{F368270}

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12384