TBA: "Request desktop site" should use desktop Tor Browser user agent

With the desktop site option enabled, the user agent is "Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Firefox/60.0". Replacing the X11/Linux part with the Windows one might cause issues on some sites, but I don't think it's expected for sites to work better with it in anyway, so it should be repurposed as a "blend in with desktop TBB users" button (maybe the option name should be changed to something along those lines, too.)