Make copied tags editable again after breaking the upstream link to library content [FC-0076]

[pomegranited]

Description

When deleting an upstream library block, ensure that any tags that may have been copied to downstream blocks are made editable again. This is achieved by un-setting the is_copied flag on the downstream tags.

This change affects Content Authors using linked library content.

Supporting information

Part of: openedx/modular-learning#244
Based on: #36111 -- compare changes
Depends on: openedx/openedx-learning#276
Private-ref: FAL-4008

Testing instructions

Note: There's a bug in the Authoring MFE's Course Unit page that prevents the tag drawer from opening. So ensure you have disabled the contentstore.new_studio_mfe.use_new_unit_page waffle flag to use the legacy Studio unit interface during testing.

0. Run migrations to add new link tables.
1. Create a block in a content library, and add some tags to it.
2. Publish the block.
3. Copy the library block into a course, either using the staged content copy/paste, or by adding a Library Content block, and selecting your published block.
   Should see the tags copied from the library block on the new course block, and they should not be deleteable.
4. Add some more tags to the new course block.
   These tags should be deletable.
5. Go back to your content library block, and delete it.
6. Return to the course block, and re-open the "manage tags" sidebar.
   All the tags should remain, and all are deletable now.

Deadline

None

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[navinkarkera]

@pomegranited Works as expected 💯 Just left a suggestion about using the event to handle the tags, let me know if you disagree.

[navinkarkera]

We can handle inside the LIBRARY_BLOCK_DELETED signal handler (preferably to a celery task) and avoid cms import inside openedx/core. Maybe add the signal handler in cms/djangoapps/contentstore/signals/handlers.py.