Use pht()

Summary:
This is the first step in Phabricator internationalization.
It adds a translation selector and calls it at startup.
Installations can add custom selectors to override some texts.
We can add official translations in future.

Next step is to allow user to choose his translation which will override the global one.

This is currently used only for English plurals.

Test Plan: Displayed a diff with unit test error, verified that it says 'Detail' or 'Details' and not 'Detail(s)'.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1139

Differential Revision: https://secure.phabricator.com/D2753