[Task] Detect UnifiedPush endpoints that rate-limit matrix.org and add a warning

There are some UnifiedPush endpoints (like `ntfy.sh`) that heavily rate-limit matrix.org (see https://github.com/binwiederhier/ntfy/issues/144#issuecomment-2098507527). In those cases, everything is set up correctly, but no notifications will arrive in the client.

Since we have no way to locally know a client is being rate-limited there is no way to detect this in real time, but we can have a sort of 'black list' of these public endpoints that we know rate-limit the homeserver and suggest the user to change this value in the same way we suggest they disable battery restrictions or enable full screen intents, with some banner in the room list.

An example of the setting they need to change: