Landing page open-enrollment redesign

[NickK21]

Summary

• updates the logged-out landing page to match the new Figma layout
• adds the teacher account CTA linking to the Google Form in a new tab
• adds the Join open-enrollment courses section to both logged-out and logged-in home views
• shows all open-enrollment courses with course title, language, and ordered module names
• adds language filtering and title/newest sorting for open-enrollment courses
• preserves join intent through login and sends users into the student course view after joining
• allows the landing-page data route to be accessed without authentication

Closes #157

[DAC098]

nit picks and questions

[NickK21]

Ready for re-review @DAC098