Remove stunprotocol.org as default for proxies and clients

The STUN server we use as a default gets hit with millions of requests/day making it costly for the operator. Checklist for rolling out these changes:

• Update the code (!129 (merged))
• Update the default bridges in tor browser (tpo/applications/tor-browser-build!617 (merged))
• Update the suggested bridge lines given out by the circumvention settings API
• Update the proxy docker container
• Update the debian package
• Update orbot bridge line (merged in https://github.com/guardianproject/orbot/commit/07e416d3a9475caf17ebb46bd437d235a440206f, but not shipped)