linkvalidator: improve tolerance for forbidden HEAD requests
Currently the linkhandler will first try a HEAD request to check an external link.
If the server returns the correct response code (405 - Method not allowed) it will retry the request using the GET method.
Unfortunately I stumbled over some misconfigured servers that will return 403 - Forbidden response codes when the HEAD method is used. In that case the linkvalidator will report a broken link, even though the GET request would work fine.
Of course it would be nice if all servers are configured correctly but there probably will always be some that are not.
So my suggestion is to also retry the GET request on 403 errors or alternatively make the response codes configurable that trigger a fallback GET request.
[TASK] Retry request on 403 in linkvalidator
When the linkvalidator detects a 403 error when doing a HEAD request to
an external URL it will retry the request using the GET method.