MDL-72984 admin: make support email setting mandatory

lib/adminlib.php

@@ -4838,6 +4838,29 @@ class admin_setting_sitesettext extends admin_setting_configtext {
 }
 
 
+/**
+ * This type of field should be used for mandatory config settings.
+ *
+ * @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+class admin_setting_requiredtext extends admin_setting_configtext {
+
+    /**
+     * Validate data before storage.
+     *
+     * @param string $data The string to be validated.
+     * @return bool|string true for success or error string if invalid.
+     */
+    public function validate($data) {
+        $cleaned = clean_param($data, PARAM_TEXT);
+        if ($cleaned === '') {
+            return get_string('required');
+        }
+
+        return parent::validate($data);
+    }
+}
+
 /**
  * Special text editor for site description.
  *