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.

You'll find install instructions in documentation (see doc/installation.rst).

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