app manager (docker/weblate): use new `files` feature to set `settings-override.py`: This file is used to set `USE_X_FORWARDED_HOST`, which is necessary to make Weblate work correctly being an Apache (and potentially other HTTP servers) reverse proxy (otherwise we have CSRF errors, cf. https://github.com/WeblateOrg/weblate/issues/1912).

this certificates are used to enable TLS during tests. Those must be the same as in
backend_e2e (they are generated there)