Decouple Channels from Daphne

[carltongibson]

• Folks want use Channels without Daphne.
• Other folks want a simple story to get up and running, which doesn't need you to be a Python packaging expert.

Historically we've favoured the 2nd of those. But keen to think if we can solve the problem.

Requirements:

1. The code and packaging changes.
2. The tutorial/docs steps spelt out clearly.

References:

• Lazy daphne import #1353 was looking at this. Noting here because it got closed renaming the main branch.
• Using uvicorn + gunicorn as ASGI server fails to boot #1639 and cap Twisted dependency below v21 daphne#359
• ... plenty of others ... (gather and add)

[carltongibson]

Looking to resolve this in #1897.

[carltongibson]

This is done and available in 4.0.0b1.

See #1898

[Archmonger]

Since this feature was completed a while ago, it should probably be unpinned.

[carltongibson]

Thanks @Archmonger 😅 Yep!