Spike if we can trivially switch notification defaults

[nadonomy]

We put a bunch of notification work on ice last year. Until we return to it, we should spike if we can trivially (on synapse?) switch the default notifications in rooms to 'Mentions & Keywords':

[ShadowJonathan]

Would this only apply to public rooms, or also to DMs and private group chats?

[t3chguy]

Given that push rules differentiate DMs as rooms with exactly 2 members, probably all rooms with >=2 members - https://matrix.org/docs/spec/client_server/r0.6.1#m-rule-room-one-to-one

[t3chguy]

Duplicate of element-hq/element-meta#1359