Impl/ezp 21678 make unimplemented rest features return a 501

The story here was to be able to have a 501 error instead of 404 on unimplemented REST features like GET content/view and GET content/view/<id>. To be able to get that we had to create a NotImplementedException value object visitor, and add routes for content/view and content/view/<id>
It's unit tested.

JIRA link : https://jira.ez.no/browse/EZP-21678