allow overriding label of links in annotation properties using rdfs:label

[jamesamcl]

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CC @dosumis, @hkir-dev

Context:

We are in the process of adding links from ontology terms and individuals generated to related content at the Allen Brain Atlas. [..] It makes no sense to generate OWL entities with labels for these types of links. Would best practice here be to provide a block of hyperlinked text in html? Would that be supported by OLS4? More generally, does OLS4 support other options for turning bare IRIs into hyperlinked text besides making named OWL entities with labels?

This PR provides a simple solution by using the rdfs:label annotation-on-annotation as the label for the link instead of the link href itself.