Emit comprehensive events for tagging/taxonomy changes

[bradenmacdonald]

Some tagging-related events were introduced in #530 and #571 , but I think we need to make the events more comprehensive. Here is a proposed spec.

Proposed specification:

1. When a taxonomy is created, a TAXONOMY_CHANGED event is emitted.

2. When a taxonomy is created via import, a TAXONOMY_CHANGED event is emitted. The
   event specifies that its tags have changed, but doesn't need to include the full
   list of tags.

3. When a taxonomy is renamed, a TAXONOMY_CHANGED event is emitted,
   and CONTENT_OBJECT_ASSOCIATIONS_CHANGED events are emitted (asynchronously) for
   every object tagged using that taxonomy's tags.

4. When a taxonomy's export_id is changed, the cached ObjectTag._export_id value is
   updated for all associated ObjectTags, a TAXONOMY_CHANGED event is emitted, and
   CONTENT_OBJECT_ASSOCIATIONS_CHANGED events are emitted (asynchronously) for
   every object tagged using that taxonomy's tags.

5. When a taxonomy's description is changed, a TAXONOMY_CHANGED event is emitted.

6. When a taxonomy is deleted, a TAXONOMY_CHANGED event is emitted, specifying that
   the taxonomy has been deleted and that its tags have changed. A
   CONTENT_OBJECT_ASSOCIATIONS_CHANGED event is emitted (asynchronously) for every
   object that had been tagged using that taxonomy.

7. When a taxonomy is enabled/disabled, a TAXONOMY_CHANGED event is emitted,
   specifying that the taxonomy has been enabled/disabled and that its tags have
   changed. A CONTENT_OBJECT_ASSOCIATIONS_CHANGED event is emitted
   (asynchronously) for every object that had been tagged using that taxonomy.

8. When a tag is renamed, the cached ObjectTag.value is updated for all associated
   ObjectTags, a TAXONOMY_CHANGED event is emitted (specifying that tags have
   changed), and CONTENT_OBJECT_ASSOCIATIONS_CHANGED events are emitted
   (asynchronously) for every object tagged using that particular tag or any of its
   children.

9. When a tag is re-parented (moved), the cached ObjectTag.value is updated for all
   associated ObjectTags, a TAXONOMY_CHANGED event is emitted (specifying that tags
   have changed), and CONTENT_OBJECT_ASSOCIATIONS_CHANGED events are emitted
   (asynchronously) for every object tagged using that particular tag or any of its
   children.

10. When a tag is created, a TAXONOMY_CHANGED event is emitted. The
    event specifies that its tags have changed.

11. When a tag is deleted, the associated ObjectTags are deleted, a TAXONOMY_CHANGED
    event is emitted (specifying that tags have changed), and
    CONTENT_OBJECT_ASSOCIATIONS_CHANGED events are emitted (asynchronously) for
    every object tagged using that particular tag. PR 571

12. When a taxonomy is updated (via re-import) a single TAXONOMY_CHANGED event is
    emitted (specifying that tags have changed), and
    CONTENT_OBJECT_ASSOCIATIONS_CHANGED events are emitted (asynchronously) for
    every object tagged using tags that were moved/renamed/deleted.

13. When copy_tags() is used to copy tags from one object to another,
    CONTENT_OBJECT_ASSOCIATIONS_CHANGED events are emitted (asynchonrously) for the
    destination object.

14. When tags (ObjectTag) are added/removed to an object (like a Component or a Container), a CONTENT_OBJECT_ASSOCIATIONS_CHANGED event is emitted. (This is currently implemented in the platform but should be moved into openedx-core so it works regardless of which API is used.)