Error 500 on competitions with keys

[madclam]

When trying to reach a competition with key e.g.
https://www.codabench.org/competitions/787/?secret_key=f21e43be-c0a4-4fa3-862e-f3ac58915257
we get an error 500 if we are not already logged in.
It should ask us for logging in and redirect to the right page.

[Didayolo]

On my side I get a 404 error using this URL

[Didayolo]

Fixed by #845 anyway