Move People to SearchFields

Summary:
Ref T8441. Ref T7715. This is the second of three ApplicationSearch + CustomField use cases (Maniphest is the third).

Also add a way to set a default ordering for the fields.

Test Plan:

• Performed searches with each field.
• Added a custom field and searched for it.
• Observed desired ordering.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7715, T8441

Differential Revision: https://secure.phabricator.com/D13190