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
