Modernize ManiphestTask paging and ordering

Summary:
Ref T7803. The ApplicationSearch integration is still a little rough here, but it seems to have the correct behavior.

The rest of this is now at least relatively sane, cohesive, and properly behaved.

Test Plan:

• Used all grouping and ordering queries in Maniphest. Pagingated results.
• Used custom field ordering in Maniphest. Paginated results.
• Paginated through the null section of "Assigned" and "Projects" group-by queries. Pagingation now works correctly (it does not work at HEAD).
• Ran unit tests covering priority changes.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7803

Differential Revision: https://secure.phabricator.com/D12372