Option to assign an inbox and annotationService to annotations

[csarven]

When an annotation is created through the dokieli UI, it should offer a way to define (or select) an inbox URL where it can receive notifications. Similarly, the annotation itself can have its own annotation service.

[csarven]

The annotation UI now (circa 2019-01) shows a checkbox for Inbox if the user specifies a location to receive announcements. The location is picked up from user's type index registry. See also https://gitter.im/linkeddata/dokieli?at=5c2cc7dc09b8066849a2625c .