Fixes #6156 Redirect /index.php/.whatever?... to /index.php?...

See #6156. `/index.php/.html?...` URLs can be interpreted by old browsers using the extension instead of the Content-Type header. This redirection should prevent that.

The part that does the redirection is in the `FrontController` (just before the dispatching). If there is a better place for this (to avoid crowding up the front controller) let me know.

I have also added a simple PHPUnit test for this. I ended up using `curl` manually because the `Http` class doesn't return the HTTP response headers. If I missed how to do that properly with `Http`, let me know too.
OK so waiting for @tsteur's PR refactoring the tests (which would add "system tests"). Then I will rebase this PR and update it so that it has:

- a separate "redirect" class with the regexes
- a unit test for the above class
- a system test so that we make sure that the redirect really works and the XSS is fixed