Add an option to lazy load room message at initial sync.

Reduces initial sync time. Useful if you joined a lot of rooms and have
a slow homeserver.

qml/form/SettingGeneralForm.qml

@@ -0,0 +1,13 @@
+import QtQuick 2.9
+import QtQuick.Controls 2.2
+
+Page {
+    property alias lazyLoad: lazyLoadSwitch.checked
+
+    Column {
+        Switch {
+            id: lazyLoadSwitch
+            text: "Lazy Load at Initial Sync"
+        }
+    }
+}