UI can't manage some segments

[emyller]

Originally, we conceived the segment rules data model so that the frontend would only send a sandwich-like ruleset, in which:

• The root rule has no conditions; rather, it's an ALL rule housing subrules.
• Subrules of the root rule must own conditions, but could have no subrules.

The API wouldn't imply any restrictions to the shape of a ruleset, so none of these matter when managing segments via the API. However, this means that our UI cannot manage some segments our customer created/updated via the API, because of being incompatible to their ruleset shape.

Some interesting timeline to keep in mind:

• Move segment create/update logic back to core #5769 — SegmentSerializer skips any rules nested deeper than a second level — which arguably makes it worse, as there's no error raised to inform the requester.
• Support segment rules top-level ANY rule for OR conditions between AND groups #7400 — The frontend was made compatible with ANY root rules.
• Store segment rule JSON alongside existing models #7815, Stop using the SegmentRule and Condition models #7818 — Segment.rules became a JSON field, which greatly simplified the multiple parts of the system dealing with segment rules. SegmentSerializer was improved to reject rules nested too deep, and typing was improved a notch.

The goal of this issue is to yet again simplify segments.

Acceptance criteria

• The frontend can manage segment rules and their conditions given any shape — it might need a major redesign of managing rules.
• The backend loses all logic that restrict or imply that rules cannot be nested, or cannot have conditions, at any nesting level, in all segment write paths.