WIP - Bug 1082444 - Use web activities to display FxA terms/privacy page...

...s.

  * We need to dismiss FxA when the terms/privacy pages are displayed, which is
    the same behavior as needed to complete the password reset, so we adopt the
    same approach. This has the nice side effect of switching from the
    non-standard EntrySheet to standards track web activities, and paves the
    way for FxA to be removed from the system app someday.
  * Moved the terms/privacy URLs into settings, so that QA can test on staging
    environments, if needed.

  Still TODO: update tests; make sure it works.
so, if we have a function that loads the url prefs, I guess this becomes

    var pref = 'identity.fxaccounts.terms.url';
    lazyLoad(pref, function(url) {
      if (!url) { console.error('oops') } // or move this inside that function
      loadExternalPage(url);
    });

and lazyLoad would be something like

    function lazyLoad(pref, cb) {
      if (!pref) { console.error('called lazyLoad with no params') }
      SettingsHelper(pref).get(function(result) {
        if (!result) { console.error('failed to load pref: ' + pref) }
        cb(result); // null result tells callers to bail
      });
    }

I should just go write code in vim. This textarea is slow to navigate :-P