Refactor standalone Access Invitation specification

[justinwb]

Access Invitations will be split into their own specification via #191, but there's a significant amount of refactoring needed on Access Invitations not only to make them sensible in their own spec, but also to align with all of the changes made to Access Consent in general.

[elf-pavlik]

We should also figure out if we will add extra terms to the interop: namespace or use a dedicated namespace. If we use interop:, eventually we'll need some automation to merge parts of the vocab together.