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