Model classes should not contain any logic

[alexandrevryghem]

Describe the bug
Currently some components have methods in their classes. This can cause issues when nested object are copied from the SSR cache to the CSR cache. We recently had this happen for the MetadataValue model class. This can easily be fixed by moving that logic to a service and calling that service with the object instead.

Expected behavior
Those methods should be moved out of these model classes into new/existing services