To make changing the gunicorn configuration more easily let's move
configuration values from systemd service to separate configuration
file.
The file will live in /etc/mailman3/gunicorn.conf.py.
To be able to enable timer it needs to have WantedBy in [Install] section. Let's
add a hyperkitty.target to that section.
Signed-off-by: Michal Konecny <mkonecny@redhat.com>
