MDL-40848 badges: Validate backpack connection via Persona

Instead of trusting that the user owns the email address they
supply, we now require them to login via Persona before
connecting them to the backpack

badges/lib/backpacklib.php

@@ -26,6 +26,12 @@
 
 defined('MOODLE_INTERNAL') || die();
 
+/*
+ * URL of backpack. Currently only the Open Badges backpack
+ * is supported.
+ */
+define('BADGE_BACKPACKURL', 'http://backpack.openbadges.org');
+
 global $CFG;
 require_once($CFG->libdir . '/filelib.php');