Bug 40597: Implement TorSettings module

- migrated in-page settings read/write implementation from about:preferences#tor
  to the TorSettings module
- TorSettings initially loads settings from the tor daemon, and saves them to
  firefox prefs
- TorSettings notifies observers when a setting has changed; currently only
  QuickStart notification is implemented for parity with previous preference
  notify logic in about:torconnect and about:preferences#tor
- about:preferences#tor, and about:torconnect now read and write settings
  thorugh the TorSettings module
- all tor settings live in the torbrowser.settings.* preference branch
- removed unused pref modify permission for about:torconnect content page from
  AsyncPrefs.jsm

Bug 40645: Migrate Moat APIs to Moat.jsm module