model rework exploration + trip creation bugfixes

[heyhuyen]

• HomeActivity - refreshMyTrips is only called once the logged in user is determined, and after trip creation
• ACL issues when creating a new trip after reopening the app (see https://github.com/ParsePlatform/Parse-SDK-Android/issues/499): used the hacky solution of setting the ACL keys.
• Fetching story places with a given trip id: we don't need to fetch the trip, we can just pass the trip id like so using ParseObject.createWithoutData:

  public static void getPlaces(String tripId, FindCallback callback) {
      ParseQuery<StoryPlace> storyQuery = ParseQuery.getQuery(STORY_PLACE_CLASS_NAME);
      storyQuery.whereEqualTo(ParseModelConstants.TRIP_KEY,
              ParseObject.createWithoutData(Trip.class, tripId));
      storyQuery.findInBackground(callback);
  }