[Tagging] Clean up Tagging models

[pomegranited]

Story

"As an Open edX developer, I want the Open edX Tagging and Content Tagging models to be as clearly delineated and simple as possible, so that developing new tagging use cases is straightforward."

Description

While implementing #63 and #77, we ended up using custom Taxonomy and custom ObjectTag classes to achieve the goals of the tagging MVP. There was a lot of back-and-forth on these PRs and changes in direction, and while the result works fine, it could be cleaner.

See comments:

• System defined taxonomies openedx-learning#67 (comment)
• System defined taxonomies openedx-learning#67 (comment)
• System defined taxonomies openedx-learning#67 (comment)
• System defined taxonomies openedx-learning#67 (review)
• System defined taxonomies openedx-learning#67 (comment) (inspiration, not a directive that must be addressed)
• Autocomplete tags API function openedx-learning#59 (comment) (re APIs)

Completion criteria

• Read the above referenced comments, and address what you think is reasonable here.
• Review the use of the Taxonomy and ObjectTag subclasses in oel_tagging and content_tagging, and refactor to ensure that the encapsulation makes sense for these models.
• Use mixins wherever practical to make logic available when subclassing.
• Review all the model get and filter/exclude clauses used in oel_tagging and content_tagging, and ensure that the appropriate indexes and unique_together constraints are in place.
• Review and update ADRs to ensure they reflect the approach decided here.

Documentation improvements

• Updating the ADRs should be sufficient documentation for this change, but also ensure that any changes to classes or their methods is reflected in their docstrings.

Relevant repositories

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

[rpenido]

Clean duplicated rules with same implementation:

https://github.com/openedx/openedx-learning/blob/f73adefbcf9748dafe8ebfb7e8e6590df6780bb2/openedx_tagging/core/tagging/rules.py#L71

https://github.com/openedx/edx-platform/blob/98fda4110056077fbdd325f325ad8fcf76460124/openedx/features/content_tagging/rules.py#L113

Done here:

• feat: implement tag object rest api in cms edx-platform#33181

[ChrisChV]

@pomegranited @bradenmacdonald I think we can close this issue