Allow custom fields to be ordered ascending or descending

Summary:
Fixes T6864. This creates a sort of busy menu but I think that's proably fine -- users are opting into activating these fields for search anyway.

In the future, we could refine this as, e.g.:

• don't show these options in the dropdown;
• do show them on some new "http prefilling" sort of page;
• then you access them as an advanced user with ?order=secret-magic.

But I'm not going to bother for now.

Test Plan: Ordered by an int field, then reversed the order.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T6864

Differential Revision: https://secure.phabricator.com/D14800