Add Access Handler class/annotation to Metadata Display Entities

[DiegoPino]

What?

We are right now inheriting the default access handler for generic Content Entities. There is nothing generic about Metadata Displays and we should provide for better future-proof compatibility/needs a custom Access handler class. A good example is \Drupal\node\NodeAccessControlHandler

This also allows us to implement more fine grained permissions for revisions and unrelated, maybe also handle the strange use case that a metadata display was created by a user, and then the user is deleted.

thx