plugin XEP-0045: user normal workflow for history + `get_room_user_jid` method: - workflow is now using the normal messages workflow even for history message: previous workflow was dedicated to MUC and only adding message to history, resulting in other plugins being skipped. - new `get_room_user_jid` helps to retrieve easily the JID currently used in a joined room.

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