Introduce translator

Summary:
This allows simple sprintf()-like translations with support for selecting variants based on numbers.

It can be fairly easily extended to support other variants (e.g. based on sex).

Test Plan: New tests.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1139

Differential Revision: https://secure.phabricator.com/D2744