Flutter and Phoenix channels

Flutter/Dart currently only has one Phoenix client library so we have two approaches to integrate Flutter and Phoenix:

+ Use the existing library: https://github.com/mfeckie/phoenix_wings
This is probably the easiest option, but we will run into some bugs/edge cases, such as heartbeat `ack`s that aren't
implemented by this library so we may have to make changes anyway.

+ Run our own on top of Darts [web_socket_channel](https://pub.dev/packages/web_socket_channel)
phoenix.js `cloc`s in at around 800 lines of OO JavaScript code, so it wouldn't be a huge task to port this over to dart.
This would provide us with a consistent API with the official library. I'm happy to do this as a "learn dart" exercise