Add "pht_list()", a translation wrapper for lists of items

Summary: Ref T13603. This is just a small piece of cleanup I've wanted to do for a while: different languages might have different list separators and repeating this implosion manually all over the place is a bit ugly even if the beahvior is never a function of translation language.

Test Plan: See next change.

Maniphest Tasks: T13603

Differential Revision: https://secure.phabricator.com/D21814