As explained in T281425#7044191, we override the default Mailman3 templates with our own that provide links to the web UI. Currently they're only in English, we should provide translated versions.
The current plan is to have the text in banana format for TWN and then convert it into text files when building it as a deb. The deb will install the templates to /usr/share/mailman-templates and we'll have puppet symlink /var/lib/mailman3/templates/domains/${host}/ to it.