Add a date column to Paste

Summary:
Show modified date on the right of the list view (which I guess
is also the created date, since pastes are immutable?)

Test Plan: Browse through the many volumes of untitled masterworks.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1600

Differential Revision: https://secure.phabricator.com/D3145