Add Terms of Service popup.

Creates a ToS popup.

• Adds a 'popups' folder to store extension popups.
• Adds a terms-of-service.html popup.
• Sets the ToS popup as the default browser action.
• Add behaviour to store ToS state, (null, true, false)
• Sets popup to open on browser extension install or update.
• Adds necessary permissions in manifest.json

Closes T378107