feat(feature flags): copy cohorts linked to a flag

[jurajmajerik]

Problem

When copying a feature flag to another project, the copying fails if the linked cohort doesn't exist in the target project.

Changes

Before copying, create any missing cohorts in the destination project.

To check if the cohort exists in the destination project, we search for the first cohort that matches by name. While it's possible there could be multiple cohorts with the same name, the user should resolve these duplications manually anyway (perhaps worth showing a warning prompt in the UI).

I went through the trouble of sorting linked cohorts by their creation order (post-order DFS), only to realize that the current implementation already returns ids in their creation order. This is because cohort ids are auto-incrementing, and converting the cohort_ids set into a list sorts them in the ascending order, which is, in fact, the order we want.

However, I'm unsure if this is something to be relied upon - perhaps there's an edge case I haven't thought of yet. That said, the current tests pass even when sort_by_creation_order is set to False.

How did you test this code?

API test, 👀

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[neilkakkar]

Almost there, good tests, good job!

Can we move the seen_cohort_cache type change to a separate PR though, it's becoming a bit too big and this one's risky on its own, since it changes high volume endpoints