Convert node type / what, when, who fields from simple configuration to config entities

While sequences can be elegant for a single field matched to content type, we have several and the who field can be compound. This should be moved to a separate form, and use a config entity schema approach.

    node_types:
      type: string
      label: 'ARK content types'
    when_field:
      type: sequence
      label: 'Field to use for when'
      orderby: key
      sequence:
        type: string
        label: 'Content type'

This will help greatly with ✨ An administrator only sees Who, When fields to set for selected Active