Add udacity course Offline Web Applications (#6169)

* Add udacity course Offline Web Applications

* Move entry to correct position

* Add author info
Co-authored-by: NDavid Ordás <3125580+davorpa@users.noreply.github.com>
Co-authored-by: NDavid Ordás <3125580+davorpa@users.noreply.github.com>

courses/free-courses-en.md

@@ -471,6 +471,7 @@
 * [learn:query](https://learnquery.infinum.co)
 * [Namaste 🙏 JavaScript: An In-Depth JavaScript Fundamentals Course](https://www.youtube.com/playlist?list=PLlasXeu85E9cQ32gLCvAvr9vNaUccPVNP) - Akshay Saini (YouTube)
 * [npm - Mastering the Basics](https://www.udemy.com/course/npm-mastering-the-basics/) - Vishwas Gopinath (Udemy)
+* [Offline Web Applications](https://www.udacity.com/course/offline-web-applications--ud899) - Google, Michael Wales (Udacity)
 * [Programming Foundations with Javascript, HTML and CSS](https://www.coursera.org/learn/duke-programming-web) - Owen Astrachan, Robert Duvall, Andrew D. Hilton, Susan H. Rodger (Coursera)
 * [Understanding RxJS](https://youtube.com/playlist?list=PL55RiY5tL51pHpagYcrN9ubNLVXF8rGVi) - Academind
 * [Vanilla JavaScript](https://www.youtube.com/playlist?list=PLillGF-RfqbbnEGy3ROiLWk7JMCuSyQtX) - Brad Traversy, Traversy Media (YouTube)