Implements the following behind an experimental configuration flag:
* A new push rule kind for mutually related events.
* A new default push rule (`.m.rule.thread_reply`) under an unstable prefix.
This is missing part of MSC3772:
* The `.m.rule.thread_reply_to_me` push rule, this depends on MSC3664 / #11804.
Part of #9744
Removes all redundant `# -*- coding: utf-8 -*-` lines from files, as python 3 automatically reads source code as utf-8 now.
`Signed-off-by: Jonathan de Jong <jonathan@automatia.nl>`
Loading push rules now happens in the datastore, so we can remove
the methods that loaded them outside the datastore.
The ``waiting_for_join_list`` in federation handler is populated by
anything, so can be removed.
The ``_get_members_events_txn`` method isn't called from anywhere
so can be removed.
