Explain a rate limiting push gateway in the notification troubleshooter (#7448)

* Explain a rate limiting push gateway in the notification troubleshooter

The push loop back test printed whatever HTTP status the gateway
answered, so a user throttled by the public ntfy.sh gateway was told
"Error: HTTP 507", which reads as a server fault rather than as a quota
they can wait out or move away from.

Map the two statuses that mean the notification was throttled onto a
typed failure and give the troubleshooter a message that says what to do.

* Import strings from localazy, remove `temporary.xml` in `:libraries:push:impl`

---------

Co-authored-by: Jorge Martín <jorgem@element.io>

libraries/push/api/src/main/kotlin/io/element/android/libraries/push/api/gateway/PushGatewayFailure.kt

@@ -10,4 +10,9 @@ package io.element.android.libraries.push.api.gateway
 
 sealed class PushGatewayFailure : Exception() {
     class PusherRejected : PushGatewayFailure()
+
+    /**
+     * The gateway refused the notification because too many were sent to it.
+     */
+    class RateLimited : PushGatewayFailure()
 }