mailman3: Add missing mailman-hyperkitty.cfg.j2 file

Fixes: 9294828f ("Setup mailman3 server")

roles/mailman3/templates/mailman-hyperkitty.cfg.j2

+# This is the mailman extension configuration file to enable HyperKitty as an
+# archiver. Remember to add the following lines in the mailman.cfg file:
+#
+# [archiver.hyperkitty]
+# class: mailman_hyperkitty.Archiver
+# enable: yes
+# configuration: /path/to/here/mailman-hyperkitty.cfg
+#
+
+[general]
+
+# This is your HyperKitty installation, preferably on the localhost. This
+# address will be used by Mailman to forward incoming emails to HyperKitty
+# for archiving. It does not need to be publicly available, in fact it's
+# better if it is not.
+# However, if your Mailman installation is accessed via HTTPS, the URL needs
+# to match your SSL certificate (e.g. https://lists.example.com/hyperkitty).
+base_url: http://localhost/archives/
+
+# Shared API key, must be the identical to the value in HyperKitty's
+# settings.
+api_key: {{ vault_mailman_archiver_key }}