Make it possible to show broken links for alle pages using mountpoints

[sypets]

Is your feature request related to a problem? Please describe.
Curently, it is only possible to show broken links for the current page and subpages. If a user has several mountpoints, it is not possible to (easily) see all broken links.

Describe the solution you'd like

1. Add additional item in select list to be able to select "all".
2. If this is selected: In case of admin, show all (without navigating through page tree). In case of non-admin, use mountpoints to consider all pages.

protected function getAllowedDbMounts(): array
    {
        $dbMounts = (int)($this->getBackendUser()->uc['pageTree_temporaryMountPoint'] ?? 0);
        if (!$dbMounts) {
            $dbMounts = array_map('intval', $this->getBackendUser()->returnWebmounts());

            return array_unique($dbMounts);
        }

        return [$dbMounts];
    }