securedrop-proxy origin limitation can be bypassed with redirects

SecureDrop Client is a desktop app for journalists to securely communicate with sources and handle submissions on the SecureDrop Workstation. Prior to version 1.3.1, a malicious SecureDrop Server could bypass securedrop-proxy's origin limitation by responding with cross-origin redirects. SecureDrop Server itself has multiple layers of built-in hardening, and is a dedicated physical machine exposed on the internet only via Tor hidden services for the Source and Journalist interfaces, and optionally via remote SSH access over another Tor hidden service. A newsroom's SecureDrop Workstation communicates only with its own dedicated SecureDrop Server. Version 1.3.1 fixes the issue.