Cohort segments lack `rules_data`

[emyller]

The Segment.rules_data field was introduced recently (#7815) as part of a larger effort (#7814) to stop reading from row-based rules and conditions, and instead use a pure JSON field.

However, segments added by the cohorts app only add row-based rules and conditions.

Acceptance criteria

• Segments managed in the cohorts app earn rules_data
• Any historical segment created by this app so far also earns rules_data

No-goals

• Please DO NOT introduce a service to own writing segments because of this. While a service may sound like an obvious choice towards normalisation, it will immediately become unjustified once we no longer have row-based rules and conditions (Stop using the SegmentRule and Condition models #7818), and rules_data — then renamed to just rules — is but a regular field like the others.