Describe the bug

The URL should've been shortened to something like https://modsn.us/<somehash>.

Cause

We have a short_url.php script which mediates requests to our modsn.us URL shortener service. This was previously supported by Nginx. However, as #1937 changes the reverse proxy to Traefik, we need to add back PHP support for this to work again.

Possible actions

1. Rewrite the script to not use PHP (done: Fix modsn.us URL shortening script #3212)
2. Add fastcgi support to our Traefik reverse proxy. No longer needed as we've migrated away from Docker (Migrate /website and /export from Docker to Vercel #3098)
3. Because the script takes a while to load, we need to improve the shortening UX by changing the UI (for context, see Fix modsn.us URL shortening script #3212):
   • Display the long URL while shortening happens
   • Display shortened URL once it's available
   • Allow toggling between original and shortened URLs so that 3rd party clients can use the URL without resolving the shortened URL. To be clear, parsing the share URL is not something we want to commit to supporting long-term, but let's just support it while it's convenient until we find a better way (e.g. with timetable sync: User accounts #796)
   • Or implement any variation that makes sense
   • Remove enableShortUrl feature flag once done.