Better behavior on model delete

[cartermak]

Checked for duplicates

Yes - I've already checked

Alternatives considered

No - I haven't considered

Related problems

Deleting a model causes trouble. Tables like plan allow model_id to be null, and that field on plans will be null on deleting the associated model, but the UI can't handle that. I'm sure we have similar behavior with other artifacts associated with models.

Describe the feature request

Unify the story around model deletion. Either:

1. Make it possible for all relevant artifacts (and the UI/containers that operate on them) to handle null values for their model_id FKs
2. Cascade deletion of models to all associated artifacts. I would argue this requires some really good UI modal, ideally collecting all of the artifacts that would be deleted for confirmation by the user.
3. Require some form of "migration" -- provide the user the option to delete a model and swap all associated artifacts over to a new model (#1530?).

There could also be a hybrid of 2. and 3. where a user can cascade deletions or migrate the artifacts, but may not delete a model while leaving all artifacts hanging.

[parkerabercrombie]

Related to https://github.com/NASA-AMMOS/aerie-ui/issues/1481