Fix currently broken systemd services

[rixx]

The systemd services are currently broken for multi-venv installations. The venv part should be the parametrised part (%i), but instead we're using {{ pretalx_virtualenv }}.

There's no real great solution for this, as we currently allow users to override the default value (/usr/share/webapps/pretalx{{ pretalx_instance_identifier }}/venv/). If they do so, we have no way to tell where the venv is, and if it even includes the instance identifier in its name. I suppose we will have to introduce a pretalx_virtualenv_base and move the /{{ pretalx_instance_identifier }}/venv part into vars and keep it hard-coded for this to work.