Warn users when deleting a snapshot that it is the target of a "thing"

### Description

Currently, deleting a snapshot does not inform the user of any devices that may be running that snapshot.

Therefore, simply "cleaning up" snapshots can have unexpected side effects on production, namely, if 1 or more devices are running a particular snapshot, they will (upon next check in) determine they longer have a snapshot an will revert to the starter flows.

See demo:

https://github.com/FlowFuse/flowfuse/assets/44235289/2e4bb840-a6ed-4616-b629-fd0d2f064c8d

Related consequences: https://github.com/FlowFuse/flowfuse/issues/2424

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate