matslats's sandbox: Entity_chooser

Defines two form elements, for selecting individual entities or whole segments of entities.
Currently it only does ContentEntities but I think ConfigEntities will be worth doing.
This is a natural progression from the d7 module, user_chooser.
Uses plugin architecture to interrogate a specific entity's table, and another plugin to define segments.
Then it generates an appropriate form widget, select, multiselect or autocomplete, and validates the input and returns the entity ids back to your $form_state.
Only the user entity is supported right now, with selections according to permissions and roles.