When printing timestamps on paper: use an absolute, context-free date format

Summary:
Ref T13573. Using the browser "Print" feature on pages produces "Thu, Aug 4, 12:22" timestamps which require context to interpret precisely (they don't have a year and don't have a timezone).

Instead, retain these timestamps in "screen" contexts but use "YYYY-MM-DD HH:MM:SS (UTC+X)" timestamps when printing.

Test Plan: Printed Maniphest tasks and other pages in Safari and Chrome using "?print=1" and "Print to PDF", saw absolute timestamps after this chagne in the printed documents.

Maniphest Tasks: T13573

Differential Revision: https://secure.phabricator.com/D21451