feat: Copy object tags [FC-0062]

[ChrisChV]

This PR adds:

• A function copy_tags in api.py to copy tags between objects.
• A new field is_copied in ObjectTag

Supporting information

• Github issue: https://github.com/openedx/modular-learning/issues/177
• It's used in: https://github.com/openedx/edx-platform/pull/35596
• Internal ticket: FAL-3616

Testing instructions

• Verify that the tests cover the new code.
• Follow the testing instructions in https://github.com/openedx/edx-platform/pull/35596

Deadline

ASAP

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[bradenmacdonald]

@ChrisChV I can merge this when you fix the conflicts and reply - let me know.

[ChrisChV]

@bradenmacdonald It's ready for merge.

Suggested some changes. Thinking ahead, we may also need an API to "convert" the tags if/when the link is broken. That is, if the user links some library into a course, then later breaks the link, the tags that were read-only would need to be changed to is_copied=False so that they can be edited. But we can do that in a separate PR if you prefer.

Sorry I forgot this, yes, we can do this in a separate PR