Add link to teacher account walkthrough video

src/views/teachers/faq/l10n.json

 {
     "teacherfaq.title": "Scratch Teacher Account FAQ",
     "teacherfaq.teacherWhatTitle": "What are teacher accounts?",
-    "teacherfaq.teacherWhatBody": "A Scratch Teacher Account provides teachers and other educators with additional features to manage student participation on Scratch, including the ability to create student accounts, organize student projects into studios, and monitor student comments.",
+    "teacherfaq.teacherWhatBody": "A Scratch Teacher Account provides teachers and other educators with additional features to manage student participation on Scratch, including the ability to create student accounts, organize student projects into studios, and monitor student comments. Learn more about Teacher Accounts in <a href=\"https://www.youtube.com/watch?v=7Hl9GxA1zwQ\">this video</a>.",
     "teacherfaq.teacherQuestionsTitle": "What if I have any questions or comments on Teacher Accounts?",
     "teacherfaq.teacherQuestionsBody": "If you have any questions or feedback on Teacher Accounts please send us a message through <a href=\"/contact-us\">Contact Us</a>.",
     "teacherfaq.teacherConvertTitle": "Can I convert my Scratch account into a Teacher Account?",