Improvements to linked attribute UI and reference handling

[robyngit]

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Soon attribute lists can be linked using EML <references> (#2652). There are a few more enhancements we could add to make the UI easier for users and references a little more robust.

(note: we should make a sub-issue for each of these)

• Allow unlinking a referenced attribute list

  • Add an "Unlink" button when viewing an attribute list that references another.
  • This should convert the reference into a standalone copy of the original attributes, so the user can edit them independently.

• Show more detail about linked attributes in the target entity

  • When an entity uses a referenced attribute list, improve the way we present those attributes:
    • Full detail option: Render the full list of attributes (like a normal editor), but disable editing and indicate that they are “linked from [Entity Name]”.
    • OR Minimal option: Show just the attribute names as we do now, but add tooltips with attr description and allow clicking the name to view the original source.

• Show which entities reference a given attribute list
  • When viewing a source attribute list (one that is referenced by others), display which other entities link to it.
  • This could be shown as a list or badge for context.

• Make the EML References model more robust
  • Track and handle the case where the referenced element is removed (either the backbone model or the EML DOM element). Ideally, warn the user before deleting a source element.
  • If the source's XML ID changes, update the references accordingly to avoid breaking the link.