R1:a5d23c9f3e88

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…