Add a "Last Edited" property to Wiki pages

Summary:
Ref T13164. See PHI797. The last edit is available in the page header, but it's not precise (just says "180 days ago") and a little weird (it's unusual for us to put that kind of information in the header).

Add a precise timestamp to the footer for now. I'd imagine re-examining this the next time Phriction gets some UI work and maybe trying to integrate timeline/transactions more cleanly (see also T1894).

Test Plan: Looked at a wiki page, then edited it. Saw precise "Last Edit" timestamp adjacent to "Last Author".

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13164

Differential Revision: https://secure.phabricator.com/D19560