Add sane mailman core configuration.

- Add configuration for exim to work with mailman core.
- Add configuration to enable hyperkitty in mailman core.
- Add configuration to enable mailman_hyperkitty plugin.
- Add mailman_hyperkitty plugin and ipython in the core container.

core/assets/mailman-hyperkitty.cfg

@@ -0,0 +1,9 @@
+[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.
+base_url: http://mailman-web:8000/hyperkitty/
+# Shared API key, must be the identical to the value in HyperKitty's
+# settings.
+api_key: ASmallAPIKey