sessionMode: Add back external session modes

Since commit da4238ec681c7ca, external session modes are no longer
loaded during start-up; bring them back.

https://bugzilla.gnome.org/show_bug.cgi?id=720017

js/ui/sessionMode.js

@@ -148,6 +148,7 @@ const SessionMode = new Lang.Class({
     Name: 'SessionMode',
 
     _init: function() {
+        _loadModes();
         let isPrimary = (_modes[global.session_mode] &&
                          _modes[global.session_mode].isPrimary);
         let mode = isPrimary ? global.session_mode : 'user';