Async publish consumer

[MarkLark86]

Purpose

Implement the PublishConsumer side of the new publishing system.

What has changed

• Added a new typed version of getting app config, i.e. get_config(int, "SOME_SETTING")
• Created a SingletonInstance base class, for easy re-use
• Created a load_class_from_config helper, that loads the class type based on key from app config
• Add new PublishConsumers (asyncio & celery)
• Remove old publish transmit code
• Update docs

[MarkLark86]

@petrjasek @eos87 docs can be found on https://superdesk--2840.org.readthedocs.build/en/2840/publish.html