Add a concept of default push rules

[jryans]

Riot Web has added a concept of default push rules that can help when new default rules are added and one or more of:

• the homeserver is outdated and doesn't yet know about the new rule
• the user doesn't have the push rule yet (as changes to defaults don't come down /sync)

Currently Riot Web uses this for:

• tombstone
• reactions

[jryans]

(Filing this issue to suggest that the mobile clients do the same.)

[manuroe]

Thanks for reporting this.
The issue for mobile clients is that they need to be woken up by their homeserver/sygnal so that they can sync with their HS and then trigger notifications.

I cannot see how it can work with an outdated server or push rules not updated.

[jryans]

Ah fair enough. Well, at least the reaction one seems like it would still apply, as that's a rule for ignoring a notification. But yes, for the tombstone case that adds a notification, I agree it might not be portable to mobile.