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/Dockerfile

@@ -7,7 +7,8 @@ ENV DEBIAN_FRONTEND=noninteractive
 # from the Gitlab.
 RUN apt-get update && apt-get install -y postgresql-client postfix \
     && pip install git+https://gitlab.com/mailman/mailman.git \
-	               psycopg2
+	               git+https://gitlab.com/mailman/mailman-hyperkitty.git \
+	               psycopg2 ipython
 
 ADD assets/run.sh /opt/run.sh