clean up and improve notifications

Thanks to Gomuks' new handling, we have sender displayname and avatar, and can use android's conversation notifications.

On other platforms, we just append sender name to the body, bolded if possible.

This also improves the logic on when to send notifications, as we drop notifs where `response["notification"]` is null.

We also now correctly set if notifications should play a sound or not, based on `PushNotification.sound`.

`PushNotification.image` handling might come sometime in the future, but it seems hard on Android.

lib/models/push_notification.dart

@@ -0,0 +1,44 @@
+import "package:freezed_annotation/freezed_annotation.dart";
+import "package:nexus/models/epoch_date_time_converter.dart";
+
+part "push_notification.freezed.dart";
+part "push_notification.g.dart";
+
+@Freezed(toJson: false, fromJson: false)
+@JsonSerializable(createToJson: false)
+class const PushNotification({
+  @EpochDateTimeConverter() required final DateTime timestamp,
+
+  required final String eventId,
+  @JsonKey(name: "event_rowid") required final int eventRowId,
+
+  required final String roomId,
+  required final String roomName,
+  final Uri? roomAvatar,
+  final bool isDm = false,
+
+  required final NotificationUser sender,
+  required final NotificationUser self,
+  final NotificationUser? perMessageProfile,
+
+  required final String text,
+  final Uri? image,
+
+  final bool mention = false,
+  final bool reply = false,
+  final bool sound = false,
+}) with _$PushNotification {
+  factory PushNotification.fromJson(Map<String, Object?> json) =>
+      _$PushNotificationFromJson(json);
+}
+
+@Freezed(toJson: false, fromJson: false)
+@JsonSerializable(createToJson: false)
+class const NotificationUser({
+  required final String id,
+  required final String name,
+  final Uri? avatar,
+}) with _$NotificationUser {
+  factory NotificationUser.fromJson(Map<String, Object?> json) =>
+      _$NotificationUserFromJson(json);
+}