Openedx-events

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Added
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* Store current versions of Avro schemas and add test to ensure valid evolution

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Changed
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* Fixed event_type of XBLOCK_SKILL_VERIFIED signal

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Changed
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* **Breaking change**: Remove ``signal`` argument from consume_events and make_single_consumer

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Changed
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* Made `signal` argument optional in consume_events in preparation for removal

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Changed
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* Added event type as namespace to generated Avro schemas

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Added
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* Configurable loader for consumer side of Event Bus in ``openedx_events.event_bus``.
* Added management command to load configured consumer and start worker.

Changed
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* Switch from ``edx-sphinx-theme`` to ``sphinx-book-theme`` since the former is
deprecated. See https://github.com/openedx/edx-sphinx-theme/issues/184 for
more details.