extensionSystem: Move extension loading into ExtensionManager

Now that extension loading and the extensions map are no longer shared
between the gnome-shell and gnome-shell-extension-prefs processes, we
can move both into the ExtensionManager which makes much more sense
conceptually.

https://bugzilla.gnome.org/show_bug.cgi?id=789852

js/ui/main.js

@@ -228,6 +228,7 @@ function _initializeUI() {
 
     ExtensionDownloader.init();
     extensionManager = new ExtensionSystem.ExtensionManager();
+    extensionManager.init();
 
     if (sessionMode.isGreeter && screenShield) {
         layoutManager.connect('startup-prepared', () => {