Improve the "get object tags" API [FC-0036]

[bradenmacdonald]

openedx/modular-learning#133

This PR updates the "get object tags" API so that it returns the results grouped by taxonomy, and sorted into "tree order" (just like the "get tags" API does).

I've also added a new API that can retrieve the # of tags applied to each unit for every unit on the course, in a single database query.

TODO

Before merging, check how this integrates into edx-platform.

Follow up

I could do a separate PR to allow including "available but unused" taxonomies in the result - those which don't yet have any tags applied to the object, but which the user could use to add additional tags. The permissions of that are somewhat complicated though so it's best as its own PR. Or perhaps it's fine to make the frontend construct this just by querying the list of available taxonomies directly, and appending them to its own list.

Private ref: FAL-3547

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[bradenmacdonald]

@yusuf-musleh If I make the "get object tags" API return data like this, it should make the unit tags overlay much simpler, right?

            assert response.data == {
                "problem15": {
                    "taxonomies": [
                        {
                            "name": "Life on Earth",
                            "taxonomy_id": 1,
                            "editable": True,
                            "tags": [
                                {
                                    "name": "Life on Earth",
                                    "value": "Mammalia",
                                    "lineage": ["Eukaryota", "Animalia", "Chordata", "Mammalia"],
                                },
                                {
                                    "name": "Life on Earth",
                                    "value": "Fungi",
                                    "lineage": ["Eukaryota", "Fungi"],
                                },
                            ]
                        },
                        {
                            "name": "user AUthors",
                            "taxonomy_id": 3,
                            "editable": False,
                            "tags": [
                                {
                                    "name": "User Authors",
                                    "value": "test_user_1",
                                    "lineage": ["test_user_1"],
                                },
                            ],
                        }
                    ],
                },
            }

[yusuf-musleh]

@bradenmacdonald Absolutely! This would make things much simpler. In the currently implementation there's a bunch of stitching stuff together, having this would reduce the API calls needed, especially if the taxonomies returned included taxonomies that are "available" and not only limited to ones that have their tags applied to the object (unit).

[bradenmacdonald]

@pomegranited I think this is ready for review, though I may need to make some changes once I try to integrate it into edx-platform. Haven't done that side of it yet.

[pomegranited]

Couple of nits, but this looks and works great @bradenmacdonald ! 👍

• I tested this using the Django runserver
• I read through the code
• I checked for accessibility issues N/A
• Includes documentation
• Commit structure follows OEP-0051

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