Make copied tags editable again after breaking the upstream link to library content

[bradenmacdonald]

As a course author, when I import library content into a course (whether via copy-paste or "Add Library Content..."), it will show the assigned tags as read-only. If I then break the link to the upstream library, I should be able to delete those tags.

This is a follow-up to #177

[pomegranited]

@bradenmacdonald or @navinkarkera Are there any other ways to "unlink" a downstream block other than deleting its upstream block?

[bradenmacdonald]

@pomegranited I don't think we expose any way to "unlink" a course block from the library version in the UI. On the backend it's as simple as removing the "upstream_" attribute(s) from the OLX.

@marcotuts @jmakowski1123 @sdaitzman Do you think we want to have an "unlink from library" button in the UI in the future? I just checked the relevant Figma design (?) but didn't see anything like that.

[pomegranited]

@jmakowski1123 @lizc577 @sdaitzman @marcotuts This is ready for AC testing on the sandbox.

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 tagged 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 added tags should be deletable.
   Note: something is broken on the MFE Unit page and it won't let me add/edit tags, so I've enabled the legacy Studio edit page instead, cf contentstore.new_studio_mfe.use_new_unit_page
5. Go back to your content library, and delete the tagged block.
6. Return to the course block, and re-open the "manage tags" sidebar.
   All the tags should remain, and all are deletable now.

[edschema]

Working well, thanks!