[FCL-309] Add the ability for documents to store abstract identifiers in MarkLogic properties

[jacksonj04]

Summary of changes

• New IdentifierSchema and Identifier abstract classes which define interfaces for how we think about what any given type of identifier looks like (IdentifierSchema), and how we might be able to interact with specific instances of that identifier (Identifier)
• New NeutralCitationSchema and NeutralCitation concretions of the above abstract classes, defining NCN-specific implementations of methods where necessary.
• Identifiers can compile themselves down to a URL slug, used for simplifying retrieval at any given URL.
• New methods for a Document which allow CRUD operations on its identifiers, using the above classes to turn identifier concepts into serialisations which can be stashed in the MarkLogic properties for a document.

Checklist

• I have created/updated method docstrings (if necessary)
• I have considered if this is a breaking change
• I have updated the changelog (if necessary)

Jira

FCL-309

[dragon-dxw]

I'm a touch concerned about the properties being used because they might be stale if overwritten, but that's a risk anyway at the moment.

Do we want more general properties parsing and editing? (Probably not)