Only show the "View Source" message action when dev tools are enabled

This is a very developer-oriented action, and other uses of this dialog
(such as interacting with room state events under a room debug page) is
also locked behind this option.

src/qml/Menu/Timeline/MessageDelegateContextMenu.qml

@@ -80,6 +80,7 @@ Loader {
             })
         },
         Kirigami.Action {
+            visible: Config.developerTools
             text: i18n("View Source")
             icon.name: "code-context"
             onTriggered: {