Update Tor Browser spec

Tor Browser 11.0 is coming out soon. We should update our design document to cover all the new issues that are showing up in it. Highlights are

1. Switch to rbm/tor-browser-build
2. The security slider copy update ...

The update should cover the current goals and state of the browser, and fold in all the 8.0, 8.5, 9.0, 9.5, 10.0, and 10.5 changes.