Refactor graphql.dart to use Flutter GraphQL package

As suggested in #22 by @MaskyS , there is a [GraphQL Flutter package](https://github.com/zino-app/graphql-flutter) that can be used for the code in [graphql.dart](https://github.com/sendgrid/dx-mobile/blob/master/lib/github/graphql.dart).

Acceptance Criteria:
- Modify the graphql.dart code to use the package
- The functionality should remain the same as how the app works prior to change