Use flutter_taag to add a tag list to add_activity_page

[petruzdroba]

https://www.youtube.com/watch?v=Nhk5fPKPV78
https://flutterawesome.com/flutter-tags-let-you-create-a-list-of-tags-or-insert-them-dynamically-with-inputfield/
https://pub.dev/packages/flutter_tags/changelog