MDL-73332 mustache: Fix shortentext helper

In the docs, it claims you can provide length to
the shortentext template helper as a context variable.
In reality, that is not the case.

lib/classes/output/mustache_shorten_text_helper.php

@@ -55,8 +55,9 @@ public function shorten($args, Mustache_LambdaHelper $helper) {
         $length = trim($length);
         $text = trim($text);
 
-        // Allow mustache tags in the text.
+        // Allow mustache tags in the length and text.
         $text = $helper->render($text);
+        $length = $helper->render($length);
 
         return shorten_text($text, $length);
     }