plugin XEP-0384: Fully reworked to adjust to the reworked python-omemo: - support for both (modern) OMEMO under the `urn:xmpp:omemo:2` namespace and (legacy) OMEMO under the `eu.siacs.conversations.axolotl` namespace - maintains one identity across both versions of OMEMO - migrates data from the old plugin - includes more features for protocol stability - uses SCE for modern OMEMO - fully type-checked, linted and format-checked - added type hints to various pieces of backend code used by the plugin - added stubs for some Twisted APIs used by the plugin under stubs/ (use `export MYPYPATH=stubs/` before running mypy) - core (xmpp): enabled `send` trigger and made it an asyncPoint fix 375