MDL-43230 badges: manually revoke awarded badges

A user can be manually awarded a badge, but if given by mistake an
awarded badge cannot be removed.

lib/db/access.php

@@ -2025,6 +2025,18 @@ $capabilities = array(
         )
     ),
 
+    // Revoke badge from a user.
+    'moodle/badges:revokebadge' => array(
+        'riskbitmask'  => RISK_SPAM,
+        'captype'      => 'write',
+        'contextlevel' => CONTEXT_COURSE,
+        'archetypes'   => array(
+            'manager'        => CAP_ALLOW,
+            'teacher'        => CAP_ALLOW,
+            'editingteacher' => CAP_ALLOW,
+        )
+    ),
+
     // View users who earned a specific badge without being able to award a badge.
     'moodle/badges:viewawarded' => array(
         'riskbitmask'  => RISK_PERSONAL,