plugin XEP-0384: fix trigger used for OLDMEMO: OLDMEMO was applied at the `send_message_data` trigger level, however this was missing stanza constructed without message data (which is notably the case for XEP-0308). This has been fixed by using the `send` trigger instead. As a side effect, the same code is now used for TWOMEMO and OLDMEMO, making it simpler.

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