Paste - add created time to pastes in a list

Summary: Fixes T5943. We do this differently in different places; Audit / Differential do something like this while Pholio expands the "byLine" to include a timestamp. Go with the Audit / Differential approach, as presumably having the date as a top line, easily scannable metadata is the goal here.

Test Plan: viewed a list of pastes and saw a timestamp of creation at the top.

Reviewers: epriestley, chad

Reviewed By: chad

Subscribers: epriestley, Korvin

Maniphest Tasks: T5943

Differential Revision: https://secure.phabricator.com/D10338