Move some PhabricatorSearchField subclasses

Summary: Move some PhabricatorSearchField subclasses to be adjacent to the application to which they belong. This seems generally better to me than lumping them all together in the src/applications/search/field/ directory. I was also wondering if it makes sense to rename these subclasses as PhabricatorXSearchField rather than PhabricatorSearchXField (as per T5655), but wasn't really sure if these objects are meant to be search-fields, or just fields belonging to the #search application.

Test Plan: N/A.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D13374