Infinite redirection after login #3340

To Reproduce
Steps to reproduce the behavior:

1. Open https://dev.tatoeba.org/users/foo/bar as guest.
2. You get redirected to the login page.
3. Enter some regular user credentials (not admin).
4. Browser gets redirected back and forth between two pages:
   • https://dev.tatoeba.org/fr/users/check_login?redirect=%2Ffr%2Fusers%2Ffoo%2Fbar
   • https://dev.tatoeba.org/fr/users/foo/bar
5. After a little while, the browser shows a misredirection error (on Firefox: "The page isn’t redirecting properly").

Expected behavior
The user should get redirected to a definite location, or a 404 error should be displayed.