Suppport "remove model" in automatic migration generator

[m4tx]

We should support removing models in the automatic migration generator.

The entry point for this is

cot/cot-cli/src/migration_generator.rs

Lines 413 to 416 in 2bbb096

	#[must_use]
	fn make_remove_model_operation(&self, _migration_model: &ModelInSource) -> DynOperation {
	todo!()
	}

. This will also require adding a new OperationInner variant:

cot/cot/src/db/migrations.rs

Line 460 in 2bbb096

enum OperationInner {

Note that the "removed model" operation will need to store all the data about the removed model, too, so that an implementation of Operation::backwards can be created.