sharingcheckmiddleware now handles externalshares as well

Added new annotations for the externalsharescontroller class
* @NoOutgoingFederatedSharingRequired
* @NoIncomingFederatedSharingRequired

By default both are required for all functions in the
externalSharesController.

A proper exception is thrown and then a 405 is returned instead of the
default error page. Since it is only an API endpoint this makes more
sense.

Unit tests added and updated