[Tagging] Implement content event signals

[pomegranited]

Story

"As a openedx developer, I want to use content creation/edition events, and so I could attach new tagging pipelines when content is created or published."

Full description

Follows on from #77
Prerequisite for #79

As described in the System Taxonomy discovery, we need to create events triggered by content creation and editing. The necessary contents follow the MVP requirements table. Events are needed for:

• Course creation and edition.
• Course Unit creation and edition.
• Course Component creation and edition.
• Library creation and edition.
• Library Component creation and edition.

 This doc is a good start point to know how openedx-events works.

Completion criteria

• PR against openedx-events and edx-platform with new content creation/edition events.
• PR against openedx-learning to update the automatic tagging ADR: Put openedx-filters under the rejected options and replace the decision with openedx-events.
• Automated testing must cover common paths in behavioral specification.

Documentation updates & improvements criteria

• Ensure that all events are fully documented, so that anyone using them is clear on their arguments and consequences.

Relevant repositories

• https://github.com/openedx/openedx-events
• https://github.com/openedx/edx-platform
• https://github.com/openedx/openedx-learning

[rpenido]

@pomegranited I think we can close this!