Provide a rough import/export pathway for translation files in Translatewiki format

Summary:
Ref T5267. This converts from Phabricator's internal format to Translatewiki's format, and from Translatewiki's format back to Phabricator.

Support isn't perfect, but this should get things started, at least.

Test Plan:

• Exported Phabricator translations.
• Generated some pirate translations.

Reviewers: chad

Maniphest Tasks: T5267

Differential Revision: https://secure.phabricator.com/D16810