plugin XEP-0277: various fixes + async: - `NS_ATOM` is now set at class level, so it can be accessed by other plugins - set `author` to `None` when no `name` element is found - make `data2entry` a corountine - fix incorrect use of yield in `trigger.point`

these certificates are used for demo purpose only. Those must be the same as in
backend_e2e (they are generated there)