Redactions should preserve the `notifications`/`invite` key on power levels

[turt2live]

Backstory: Notification power levels were added after redactions and hashes were introduced. This results in a protocol version bump being needed to change the redaction rules otherwise hashes would be wrong for servers, making the event fairly conflicted.

[richvdh]

Apart from for symmetry with the other power_levels settings, why is it important that redactions do not redact notifications? It doesn't make any difference to the authorization of events in the room (unlike the other power_levels settings: arguably, notification permissions should have gone in a different event type.)

[turt2live]

Because of similar reasons for matrix-org/matrix-spec-proposals#1996 (comment)

Let's keep the argument of convenience vs protocol in one place please.

[richvdh]

(discussion now moved to matrix-org/matrix-spec-proposals#2176)