Mark date and time format translatable

Summary:
This allows translations to specify custom date and time format.
We will need to call setlocale() or build system independent on it at some point but I guess it is good for now.

Test Plan: Translated 'D, M j, g:i A', saw it in diff.

Reviewers: btrahan, epriestley

Reviewed By: btrahan

CC: aran, Korvin

Maniphest Tasks: T1139, T1321

Differential Revision: https://secure.phabricator.com/D2809