if archives are unavailable, explicitly say so

Original LP Author: Sumana Harihareswara , LP Link: https://bugs.launchpad.net/bugs/1444731

Followup to #1444471.

On a Postorius list description page, e.g., https://lists-dev.cloud.fedoraproject.org/admin/lists/hyperkitty-test.lists-dev.cloud.fedoraproject.org/ , if the archive is available only to list members, we should say that. If there are no archives available (either because there's no HyperKitty installation or because the list owners choose not to archive the list), we should explicitly say that there are no archives available.

Suggested wording:

If they're only for list members:

(The current archive is only available to the list members.)

If there are no archives:

(No archives are available.)