Assign and remove project resources without unnecessary churn (Fixes: #585).

[andrewsomething]

The current implementation moves all resources in the old config to the default project and then adds the resources in the new config to the Terraform-managed project. This creates a lot of unnecessary churn.

terraform-provider-digitalocean/digitalocean/resource_digitalocean_project.go

Lines 208 to 209 in 2c4bf56

	// The API requires project resources to be reassigned to another project if the association needs to be deleted.
	// a diff of the resource could be implemented instead of removing all, (bulk) and adding the back again.

This PR adds a helper function to compares two *schema.Sets, producing lists of which should be added and which should be removed. Resources that were in both the old and new configs will no longer be touched.

Fixes: #585

[bentranter]

Looks great!