Delete orphaned approvals

I have added a hook_ENTITY_TYPE_delete() for both user and workspace entities.

On the user case, I am checking only for the reviewer.
I have not tried to check for the owner.
Arguably, even if the owner of the request approval is removed, the approval may still continue.

I also needed to add a small change to the entity class to handle the no subject case a bit more, following the recent addition to allow it to be nullable at a0d4ce4fab6a8310860a7de94936852a6198a1ec.

New commits on new 3478867-delete-orphaned-approvals topic branch and related new MR !2

- 260b87e Delete workspace request approvals when the reviewer is deleted
- e72f5e6 Handle no subject on WorkspaceApprovalRequest::urlRouteParameters()
- d9cc422 Delete workspace request approvals when the subject is deleted

Great work here.

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