Allow users to modify component's host URLs

[Archmonger]

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Description

Added

• Distributed Computing: ReactPy components can now optionally be rendered by a completely separate server!
  • REACTPY_DEFAULT_HOSTS setting can round-robin a list of ReactPy rendering hosts.
  • host argument has been added to the component template tag to force components to render on a specific host.
• reactpy_django.utils.register_component function to manually register root components.
  • Useful if you have dedicated ReactPy rendering application(s) that do not use HTML templates.

Changed

• ReactPy will now provide a warning if your HTTP URLs are not on the same prefix as your websockets.
• Cleaner logging output for detected ReactPy root components.

Deprecated

• reactpy_django.REACTPY_WEBSOCKET_PATH is deprecated. The similar replacement is REACTPY_WEBSOCKET_ROUTE.
• settings.py:REACTPY_WEBSOCKET_URL is deprecated. The similar replacement is REACTPY_URL_PREFIX.

Removed

• Warning W007 (REACTPY_WEBSOCKET_URL doesn't end with a slash) has been removed. ReactPy now automatically handles slashes.
• Warning W008 (REACTPY_WEBSOCKET_URL doesn't start with an alphanumeric character) has been removed. ReactPy now automatically handles this scenario.
• Error E009 (channels is not in settings.py:INSTALLED_APPS) has been removed. Newer versions of channels do not require installation via INSTALLED_APPS to receive an ASGI webserver.

Checklist:

Please update this checklist as you complete each item:

• [x] Tests have been included for all bug fixes or added functionality.
• [x] The changelog has been updated with any significant changes, if necessary.
• [x] GitHub Issues which may be closed by this PR have been linked.

[Archmonger]

@rmorshea This PR means reactpy-django officially supports distributed computing. Way more scalable architecture, so I'm interested to see if anyone will use it for deployment.