The suggestion my colleague posted applies to you as well. Have in mind that App_Data is an ASP.Net reserved folder and permissions to it differ from the ones applied to your site's root. Placing the js files in another folder or at root level will solve the 403 error provided the permissions for the root folder are in order.
Another thing I can add here is a reference to this documentation article
concerning registering themes inside sitefinity and this blog post
concerning the proper way to include scripts.
Ivan D. Dimitrov
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