Change HTTP status code of protected board join request

[brandstetterm]

If authenticated users want to join a protected board, they have to enter the passphrase first. The problem is that the backend returns the 403 status code on multiple occasions. On the frontend, it's hard to distinguish these responses. Therefore, the response status code of the request to join a protected (password-protected) board without a password should be change to return 400 Bad Request instead of 403 Forbidden.