Prevent deletion of an organisation that has an active Chargebee subscription

Since we added (mandatory) cancellation reasons in chargebee, deleting an organisation with an active CB subscription fails with the following error.

The desired solution (as discussed [here](https://flagsmith.slack.com/archives/C05GSDTULDB/p1780489630611909)) is to prevent users from deleting their organisation if it has an active subscription. The 'Delete Organisation' button should be disabled in the UI, and it should be clear to the user that they first need to cancel their subscription in order to delete their organisation.

Related sentry Issue: [FLAGSMITH-API-5QM](https://flagsmith.sentry.io/issues/7524030089/?referrer=github_integration)