Provide "builtin" high-level result orders

Summary:
Ref T7803. Currently, available high-level orders are spread across Query and SearchEngine classes and implemented separately for each application.

Lift the concept of "builtin" (high-level, user-facing, named) orders (similar to "builtin" queries in ApplicationSearch) into the root Query class, and let it drive the SearchEngine implementation. This allows you to define a new order in one place and have it automatically work across the entire stack.

This will also let Conduit expose this information in a straightforward way.

Test Plan:

• Used ApplicationSearch in Diffusion.
• Used all result orderings.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7803

Differential Revision: https://secure.phabricator.com/D12379