Add a phutil_person() wrapper for the string extractor

Summary:
Ref T5267. When extracting strings, we can currently statically infer information about plurality (via PhutilNumber and phutil_count) but not about gendered variables.

This adds a marker for gendered variables. Usage will be:

pht(
  '%s stood silently.',
  phutil_person($variable));

The string extractor can then annotate this as gendered during extraction.

Also, use more consistent terms for grammatical gender.

Test Plan: arc unit, see next diff.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T5267

Differential Revision: https://secure.phabricator.com/D16822