Implement MSC3772: Push rule for mutually related events

[clokep]

MSC3772 augments push rules with additional checks.

To implement we need to:

• Implement the new push rule condition.
• Add the additional default push rules:
  • .m.rule.thread_reply_to_me
  • .m.rule.thread_reply

Depends on #11804.

[clokep]

I think the new push rule condition either needs some changes to the push infrastructure to allow it to dynamically fetch more info or it needs to pull from event_relations to see what unique groups of (rel_type, sender, type); this could be a lot of information, however.

We might be able to pre-process conditions to see what we need to pull though and then only have a cache of booleans. Still sounds somewhat expensive though.

[clokep]

This is no longer planned at the moment.

[daniellekirkwood]

The team made a decision for the Exit Beta Milestone and we landed here: https://docs.google.com/document/d/1DYyDsf24OlrME3h-Hg-H2GvCrKkqZJrQwIbNWjqxB2A/edit?usp=sharing

therefore we do not need to move forwards with this issue at this stage. It's an enhancement for the future so I will mark it as such! :)