Added editor control in admin settings & Fixed some markdown editor bugs

Also updated the setting system with a more sane approach to handling default values. (Now done via the setting-defaults config file)

config/setting-defaults.php

@@ -0,0 +1,10 @@
+<?php
+
+/**
+ * The defaults for the system settings that are saved in the database.
+ */
+return [
+
+    'app-editor' => 'wysiwyg'
+
+];