Draft: fixup! Bug 27476: Implement about:torconnect captive portal within Tor Browser

This fixup patch adds two new pieces of functionality:

• hitting 'Escape' key in about:torconnect cancels an in-progress connection request, as if user had hit the cancel button
• added new extensions.torlauncher.quickstart pref (set to false by default in separate tor-launcher patch) that is configured in about:preferences#tor, and in the about:torconnect page