WYSIWYG descriptions: Allowed anchor target attrs

Allowed since this is a control in the editor UI, but would previously
be stripped by editor config & server-side filtering.
For #4925

app/Util/HtmlDescriptionFilter.php

@@ -20,7 +20,7 @@ class HtmlDescriptionFilter
      */
     protected static array $allowedAttrsByElements = [
         'p' => [],
-        'a' => ['href', 'title'],
+        'a' => ['href', 'title', 'target'],
         'ol' => [],
         'ul' => [],
         'li' => [],