Extra permission to restrict the canonical poll page

Merge request created

this is much trickier than you think because the canonical page is always used for the form submission, you will need to set up a custom request subscriber or so that only disallows GET requests.

It would also require an update function to grant that permission to users that already are allowed to see polls, and a lot of test updates as you can see.

Honestly, I'm not really interested in supporting this use case and am strongly considering to mark this as won't fix.

@Berdir that I obviously didn't think of. So the request subscriber would be the way to go?