Entity lookup doesn't respect Relationship's maximum cardinality on the opposite side

[ybnd]

Describe the bug

When a Relationship type is configured with a maximum cardinality, DSpace limits how many of those Relationships a given Entity can have at a time.

This is not reflected in our Entity lookup modals (submission or edit Item), which can lead to confusion:

• Some Entities that appear between the search results may have reached their limit already
• The submitter has no way to know which Entity can be linked until they try
• Once they do try, a generic error notification is shown

To Reproduce

Steps to reproduce the behavior:

1. Set the right_max_cardinality of isAuthorOfPublication to 1 -- this implies that a Person Entity can only have one related Publication
2. Submit a new Publication with one Author (A)
3. Submit another Publication and open the Author lookup modal again
   • You'll see Author A between the search results
   • If you attempt to link it to the new Publication, a server error notification will be shown: Unable to add relationship

Expected behavior

Two possible approaches:

1. Don't show Entities that have reached their cardinality limit in the lookup modal
   • Would require a new indexing plugin
   • Could still be confusing for someone trying to find a particular Entity, unaware of the cardinality limit
2. Show all Entities, but disable the "checkbox" for Entities that have reached their cardinality limit (possibly add an explanation as to why)