MDL-68991 core: Prevent popup blockers blocking feedback window

Some browsers like Firefox are very inflexible with window.open()
and block it if it is not instantly invoked after the user click.

Also according to https://stackoverflow.com/a/6807615 it is best
practice to replace self:: with static::

lib/classes/userfeedback.php

@@ -57,9 +57,9 @@ class core_userfeedback {
         $actions = [
             [
                 'title' => get_string('calltofeedback_give'),
-                'url' => '#',
+                'url' => static::make_link()->out(false),
                 'data' => [
-                        'action' => 'give',
+                    'action' => 'give',
                     'record' => 1,
                     'hide' => 1,
                 ],
@@ -103,13 +103,13 @@ class core_userfeedback {
             $lastactiontime = max($give ?: 0, $remind ?: 0);
 
             switch ($CFG->userfeedback_nextreminder) {
-                case self::REMIND_AFTER_UPGRADE:
-                    $lastupgrade = self::last_major_upgrade_time();
+                case static::REMIND_AFTER_UPGRADE:
+                    $lastupgrade = static::last_major_upgrade_time();
                     if ($lastupgrade >= $lastactiontime) {
                         return $lastupgrade + ($CFG->userfeedback_remindafter * DAYSECS) < time();
                     }
                     break;
-                case self::REMIND_PERIODICALLY:
+                case static::REMIND_PERIODICALLY:
                     return $lastactiontime + ($CFG->userfeedback_remindafter * DAYSECS) < time();
                     break;
             }