Create a MapsApiPlacesTextField Widget to allow users to select places for trips and placeVisits

[anfee1]

When trying to use the Maps API in flutter there is a problem with the API key. It could be that there is problem with knowing whether Flutter is acting as a web app or not, but more research should be done on the issue.

[KoopaKing]

So I did some prototyping with this, and I actually think having a Map display during the creation page is the wrong thing. What we really want is a MapsApiPlacesTextField that does autocompletion for either places associated with trips or placeVisits.

Some learnings from my prototyping:

1. we should probably be using the libraries flutter_typeahead and google_maps for this. flutter_typeahead was the best library I could find for autocompletion that seemed to be easy to use and looked good. google_maps is a wrapper around the JavaScript API, which makes the code look a little gross, but it seems to do what we need it to do for this.
2. Because google_maps wraps the JavaScript API, it doesn't seem super natural with Flutter. Specifically the API uses the pattern service.preformAction(request, (result, status) { callback... }) which doesn't gel with a lot of Flutter APIs that expect futures. To get around this you can use the Completer class as such:

// Transforms the callback-style api into a future-style api.
Future<ResultType> getResult(RequestParamType requestParam) async {
  Completer<ResultType> completer = Completer();
  final request = RequestType()
    ..param = requestParam;
  service.getResultType(request, (result, status) {
    if (status == Service.OK) {
      completer.complete(result);
    } else {
      completer.completeError("Error getting results");
    }
  });
  return completer.future;
}

3. We probably want Trips to use a Maps API Place ID to indicate their location instead of the latitude/longitude.
4. We probably want to have a static set of Maps API Places types that map to Trips and PlaceVisits, for example Trips probably should include the Maps API Places type (cities) and PlaceVisits should probably include the type tourist_attraction.
5. We should use those types as a parameter to the MapsApiPlacesTextField to ensure suggestions are relevant for the place being selected (for creating Trips and creating PlaceVisits).

[KoopaKing]

Actually the third item on the above list should probably be its own thing apart from making the autofill field.

[KoopaKing]

Created #95 to address the third item, but it should be non-blocking for this type of text box.