Rewrite old file + sitetree links to new shortcode format in HtmlEditorF...

...ield.php

The file and sitetree links shortcode formats have changed and old url based file links are no longer taken care of by HtmlEditorField.

This proposed change will rewrite those links into the form that is expected by the component.
A migration task might be better suited for this, but this is what I came up with.