Fixes: #18373 - Fix validation of site in Assign Device to Cluster flow

[bctiemann]

Fixes: #18373

Assigning a Device to a Cluster goes through a validation to ensure the Site of both objects is the same. Because Cluster is now generically scoped, this causes an error as Cluster.site no longer exists.

This change fixes the validation to compare Site objects properly, and also conforms to a pattern that can be extended to validate Location, Site Group, etc. scopes if they are added to the Assign Device form in the future.

[jeremystretch]

I think we need to validate every potential type of cluster scope (region, site group, site, or location). Ideally, this should happen under the model validation, but that might be difficult to manage given the deviation from standard patterns in this particular view.

[bctiemann]

As it turns out Device doesn't have site_group or region fields, only site and location. So I extended this validation to cover both those fields, and it means it's further a pattern outlier that isn't likely to be applicable to many other objects that aren't fully scoped.

Also note that this validation will throw an error of one of the Device or Cluster has a value specified but the other is null. Maybe we don't want that to be an error condition?