Refactor the drafts service to preserve unsaved edits
when switching groups.

This rewrites the drafts service so that it can preserve
unsaved changes for new and existing annotations when
switching groups.

For each annotation, the drafts service now maintains
an object containing the unsaved changes in addition to
the model for the annotation which was being edited.

 * For new annotations, the annotation is moved to the current
   group when switching groups.

 * For edits to existing annotations, the unsaved changes are
   saved to the drafts service.

 * When an annotation card is created, switch to editing mode
   automatically if a draft is present.

 * Avoid automatically discarding the draft when an annotation
   is unloaded. This allows unsaved edits to existing annotations
   in a group to be restored when switching back to the group.