R1:d6ec905fe30d

Allow overriding translations without creating PhabricatorTranslation

Test Plan: Overridden '%d Detail(s)', verified that it was used.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1139

Differential Revision: https://secure.phabricator.com/D2815