use computed fields instead of FieldAPI extra fields

[joachim-n]

The directory module currently uses FieldAPI extra fields (aka pseudofields) for various things that are output on Directory Channel nodes. This works by declaring items in hook_entity_extra_field_info() and then adding output to the render array in hook_ENTITY_TYPE_view().

This has some drawbacks - for one thing I think it's the cause of #358. It's also quite limited as while you can arrange pseudofields in the display output, there's no way of configuring them.

Using computed fields instead could be a cleaner and more modern way to add output to directory channel nodes. Fields are declared in an entity info hook, and then a field plugin produces the render output. Caching and use of lazy builders for output which is dynamic needs to be considered -- I don't know how the current pseudofields handle this.