Segment rule ordering is not persisted

[khvn26]

We have no way to explicitly provide segment rule order to the API when creating/updating them, nor any ordering defined when selecting them.

We should:

1. Add a numeric Rule.priority model field
2. Expose it in the API in a backwards-compatible way:

• Add default ordering = ["priority"] model setting
• Assume condition ids as priority values if no explicit "priority" keys offered in the mutating API requests.

I don't yet know how to reliably assume default values for the new field in a migration. We should test with current data/queries to come up with a path forward.

[khvn26]

Introduced a partial solution in #5671. We still need to think on a migration plan to make the setting on by default, and deprecate it over time.