web-users: make sure units start on dependent package upgrades

After the change is deployed verify the resulting units are enabled.

roles/web-users/templates/docker-web-users-http-dav.systemd.j2

@@ -2,3 +2,7 @@
 # https://git.autistici.org/ai3/prod/-/issues/244
 [Unit]
 PartOf=ai-webdav-auth-server.service
+
+# Make sure 'systemctl start ai-webdav-auth-server' also brings up this unit
+[Install]
+WantedBy=ai-webdav-auth-server.service