Add a concept of default push rules, using it for tombstone notifications

[turt2live]

Part of element-hq/element-web#8447
See also matrix-org/matrix-spec-proposals#1930

Intended for matrix-org/matrix-react-sdk#2798

[dbkr]

I guess this is fine in theory but push rules are complex enough as they are without each client making up their own layer of defaults to add into the mix. Is this because we're struggling to find resources to implement MSC1930 in synapse?

[turt2live]

Synapse implements it for new logins/users, but existing users who haven't re-logged will not have the push rule in their sync stream.

This is more to solve the problem of synapse not telling us about the new push rule and the smaller case of people using an older homeserver which does not have the push rule defined. This way, the user still receives notifications for upgrades but doesn't need to update their homeserver or re-login.

[dbkr]

right, yep - I remember this from last time we added a default rule. Back then I think we fetched the rules separately. If we can't trust what synapse is sending us down the sync stream, there's an argument for fetching them separately rather than trying to second guess what we think they probably ought to look like. That's probably more complex and almost as bad though, so maybe just put a big XXX comment here to say it's doing this because of a bug in synapse?

[turt2live]

That comment is the first line of the diff - it's less of a synapse thing and more of a "we want red notifications for tombstones, regardless of server support"