Compact the event cache JSON

By default, Qt will spit out indented JSON when writing to the config
file, which is useless as it wastes space and no one will read this.

src/neochatroom.cpp

@@ -316,7 +316,7 @@ void NeoChatRoom::cacheLastEvent()
     if (event != nullptr) {
         auto &roomLastMessageProvider = RoomLastMessageProvider::self();
 
-        auto eventJson = QJsonDocument(event->fullJson()).toJson();
+        auto eventJson = QJsonDocument(event->fullJson()).toJson(QJsonDocument::Compact);
         roomLastMessageProvider.write(id(), eventJson);
 
         auto uniqueEvent = loadEvent<RoomEvent>(event->fullJson());