Remove system-defined taxonomies, replacing with "read_only" flag

[bradenmacdonald]

Implements #634

• Remove the "system-defined" taxonomy feature entirely, replacing it with a read_only flag on the base Taxonomy class
• Remove all taxonomy subclasses and the cast() functionality
• Remove the built-in "Language Taxonomy" and "User Taxonomy" which weren't useful nor usable. (If the languages taxonomy was actually in use, it gets converted to a regular taxonomy.)
• Remove the unused object_tag_class override. We only use ObjectTag.

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[bradenmacdonald]

@ormsbee (CC @mgwozdz-unicon @jesperhodge): when you have some time, could you please take a look at this PR and the corresponding platform PR?

[jesperhodge]

I'll start by sharing Claude Code Review:
"

1. This PR renames the taxonomy API field system_defined to read_only. The Studio frontend (frontend-app-authoring) still reads system_defined: it uses it to show the "System-level" badge and to hide the Import and Delete menu items for those taxonomies. After the rename the frontend gets no value there, so the badge won't render and those menu items will start appearing for read-only taxonomies. I couldn't find a frontend PR that updates this. I think it breaks the badge and menus in Studio. Could we add a small frontend-app-authoring PR alongside this one to keep them in sync?

2. ADRs 0007 and 0008 document the system-defined-taxonomy and auto-tagging features that this PR removes, but they aren't touched here. Would you be open to amending them?

3. Migration 0021 handles the built-in "Languages" taxonomy (id -1): if it's in use it's converted to a regular taxonomy, otherwise it's deleted.

On deletion, there's a related TaxonomyOrg row for taxonomy_id=-1 (owned by content_tagging). Does deleting the taxonomy also remove that row? A test that runs the real migration against a database (rather than the pytest-built schema) would confirm it.
"

[bradenmacdonald]

Thanks @jesperhodge !

Could we add a small frontend-app-authoring PR alongside this one to keep them in sync?

Good point! Here it is: openedx/frontend-app-authoring#3183

ADRs 0007 and 0008 document the system-defined-taxonomy and auto-tagging features that this PR removes, but they aren't touched here. Would you be open to amending them?

Fixed in 4e5e09d

On deletion, there's a related TaxonomyOrg row for taxonomy_id=-1 (owned by content_tagging). Does deleting the taxonomy also remove that row?

Yes, it does. TaxonomyOrg has on_delete=models.CASCADE, and I've tested the migration on my devstack. It's also on a layer above this app, so this app doesn't need to know about it.

[jesperhodge]

Great work! Thanks so much for this, that removes a lot of cruft in front of the Competency work. The PR looks really good, I tested with the frontend PR briefly and it seems to work fine.
I'd love my suggestion for the TagPermissionsMixin addressed if possible (reject it if you don't agree), but I'm not going to block the PR on that minor detail.

[jesperhodge]

Suggestion: self.view = view here is only ever read for .get_taxonomy(), and only by TagPermissionsMixin — TaxonomyPagination never uses it. Might it simplify this by capturing the taxonomy directly there instead, and drop view/self.view from the base mixin?

class TagPermissionsMixin(CanAddPermissionMixin):
    def paginate_queryset(self, queryset, request, view=None):
        self._taxonomy = view.get_taxonomy() if view else None
        return super().paginate_queryset(queryset, request, view=view)

    def get_can_add(self, _instance=None) -> bool | None:
        return super().get_can_add(Tag(taxonomy=self._taxonomy) if self._taxonomy else None)

[jesperhodge]

Non-blocking nit: I suggest dropping the system_taxonomy language to avoid confusion later.

Suggested change

	self.system_taxonomy = Taxonomy.objects.get(name="Read Only Taxonomy")
	self.read_only_taxonomy = Taxonomy.objects.get(name="Read Only Taxonomy")

[jesperhodge]

See my comment in the other PR: openedx/openedx-platform#38895 (comment)
Might make sense to add a little change here?