25563357 moved the logout redirect logic from the client-side to the server-side. Unfortunately the login_required check was lost during the refactoring which meant that non-login-required sites would redirect to `/login` after redirect, and immediately restart the login process. Depending on the SSO implementation, that can make it impossible for users to log out cleanly.
This commit restores the login_required check, and prevents the potential redirect loop.
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