Prevent unsupported characters from being used in as subjects

So, the MR does:
* Prevent saving and loading entities when their subject contains unsupported characters.
* Prevent attempting to load entities in the router parameter converter class, when the passed URI contains unsupported characters.

I think this MR is fine as it is. The situation is actually that the module protects the backend from the entities having invalid IRIs described in the IS. The edge case is properly handled, i.e. it cannot happen via SparqlEntityStorage (neither load nor save).

Maybe the throw exception can be added to the docblock of the corresponding SparqlEntityStorageInterface methods, but that's only a nit.

The Kernel test additions are fine too.

My personal preference dictates that the method you test in ParamConverterTest (SparqlEntityStorageConverter::convert) should be set up in a way that it is able to return even an entity. So if I fix the entity definition param to be an array with the required key 'type', and then I add a valid entity to the test provider, then the test run must fail because convert returns with a non-null value. This might help to catch regressions in the future (or catch changes in upstream). But I repeat, this is only my preference. Attaching an oldschool diff as reference

Sorry, forget to set the right issue status, but keep assignee.

Automatically closed - issue fixed for 2 weeks with no activity.