Fixes ai3/prod#262

This lets Roundcube do the "remote image" enforcement, otherwise
even when users enable the functionality, it won't work, which is
quite confusing (though arguably safer).

Should fix the "Class Spoofchecker not found" error.

S6

See merge request !2

This reverts commit 40282123.

This reverts commit d650c370.

Mitigation for contacts having disappeared.

With status code 500 nginx will replace the error page with its own,
and the users won't see the message.

The redirect to the accounts URL was just confusing for users
that hit bugs with the authentication system. It's better to
be explicit about the situation.