MDL-66940 badges: Create page to display badges info

The OBv2.0 specification includes a field "Criteria" for
BadgeClass. Until now, this field was filled using the
URL of the badge assertion, but that is causing some issues
in Badgr because it linked to the badge assertion of the
first user sending this badge to the Badgr backpack (so then,
the following users linked to the first user assertion page
too).

This patch adds a new page, badgeclass.php which will be
used from now to display any badge information which is
not related to any assertion (like happens with the criteria
in BadgeClass).

badges/classes/assertion.php

@@ -196,7 +196,10 @@ class core_badges_assertion {
                 }
             }
             $class['image'] = 'data:image/png;base64,' . $imagedata;
-            $class['criteria'] = $this->_url->out(false); // Currently issued badge URL.
+
+            $params = ['id' => $this->get_badge_id()];
+            $badgeurl = new moodle_url('/badges/badgeclass.php', $params);
+            $class['criteria'] = $badgeurl->out(false); // Currently badge URL.
             if ($issued) {
                 $params = ['id' => $this->get_badge_id(), 'obversion' => $this->_obversion];
                 $issuerurl = new moodle_url('/badges/issuer_json.php', $params);
@@ -281,13 +284,15 @@ class core_badges_assertion {
     public function get_criteria_badge_class() {
         $badge = new badge($this->_data->id);
         $narrative = $badge->markdown_badge_criteria();
+        $params = ['id' => $this->get_badge_id()];
+        $badgeurl = new moodle_url('/badges/badgeclass.php', $params);
         if (!empty($narrative)) {
-            $criteria = array();
-            $criteria['id'] = $this->_url->out(false);
+            $criteria = [];
+            $criteria['id'] = $badgeurl->out(false);
             $criteria['narrative'] = $narrative;
             return $criteria;
         } else {
-            return $this->_url->out(false);
+            return $badgeurl->out(false);
         }
     }