Allow user override translation and implement PhutilPerson

Test Plan:
Altered database.
Wrote a custom translation and selected it in preferences.
Verified that the text is custom translated.
Set language back to default.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1139

Differential Revision: https://secure.phabricator.com/D2757