Add "Access Restrictions" to cluster list and to templates/text when creating cluster issues

[dguendisch]

Why is this needed: The "Access Restrictions" (https://github.com/gardener/dashboard/issues/689) are pretty important when consumers have them in place. Hence admins/operators should have a direct visual feedback when the cluster in question has access restrictions imposed.

What would you like to be added: I'd want to have a visual feedback on clusters with imposed "Access Restrictions":

• some kind of icon in the cluster list clearly showing that "Access Restrictions" are in place (ideally with some hover/popup on the details of the "Access Restrictions").
• some kind of textual hint on a github issue created directly from the gardener dashboard, e.g. "[Restricted Access!][project/shoot] bla" or a label or some hint in the issue content, so that also admin/ops that interact with github primarily are prominently made aware of access restrictions.