Push Notifications (#66)

Fixes #35.

Things to do:
- [x] Register with local distributor
- [x] Register with homeserver
- [x] Recieve messages
- [x] Show notifications
- [ ] Notifications page (could come later)

Reviewed-on: #66

lib/models/client_state.dart

@@ -10,6 +10,7 @@ class const ClientState({
   required final bool isLoggedIn,
   required final bool isVerified,
   required final String? userId,
+  required final String? deviceId,
   required final String? homeserverUrl,
 }) with _$ClientState {
   Map<String, Object?> toJson() => _$ClientStateToJson(this);