Migrate New Identity feature from torbutton to firefox

• Migrate functionality out of torbutton and into firefox
  • Implement some sort of 'hook' functionality so tor-browser can do tor-specific cleanup when New Identity feature is triggered (for example, make an observer topic that can be subscribed to)
  • Split current work done into two patches:
    • base-browser cleanup (profile cleanup, etc)
    • tor-browser cleanup (w/e tor-specific stuff happens ???)
• Ensure only depends on strings in the new localization project (see #40924 )