Add a trivial integration test which just takes a screenshot

This required refactoring the code. With this we can now write an
integration test to test the main parts of the app, but more importantly
we can automate the process of generating the screenshots.

test_driver/main.dart

@@ -0,0 +1,7 @@
+import 'package:flutter_driver/driver_extension.dart';
+import 'package:journal/app.dart';
+
+void main() async {
+  enableFlutterDriverExtension();
+  await JournalApp.main();
+}