Instances: Improved deletes

When a user deletes an resource, there's some ambiguity as to whether associated resources are also deleted. We want to make this process as explicit as possible. It would also be useful to add further friction to prevent the user accidentally deleting an instance as this is irreversible.

To begin I propose we add this to the instance delete flow, and eventually generalise it to work with other resources. E.g. deleting a project (and with it its contained resources).

We may also want the ability for the user to cleanup these resources automatically. We could do so by batching the API requests, similar to how we do it on the image upload modal.

Start by reviewing the instance delete flow and the image upload modal's batching approach, since the issue points to both as relevant precedents. Define explicit handling for associated resources, add stronger protection against irreversible deletion, and consider cleanup batching; done means the instance flow clearly communicates consequences and supports the agreed cleanup behavior.