Improve NoScript and Browser integration

• Extract Security Level backend functionality from torbutton and make into separate firefox patch
• Move Security Level UI Patch before the tor-browser split in firefox
• Ensure only depends on strings in the new localization project (see #40924 )

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• Update browser threat-model doc
• Determine what functionality we want to expose to the user
• Integrate NoScript as a system extension
• Build some system for NoSript to trigger 'native' browser UI (modals, etc)
• Implement w/e UX Team comes up with
• Allow about:addons intalled NoScript to override system-extension NoScript and disable w/e custom security features we have