Annotations: make interface cleaner for removing annotations

The method and variable names involved in removing annotations have been
updated to reflect that not only neurons can be annotated. Also the Django
API changed from '/neuron/<ID>/annotation/<ID>/remove' to
'/annotations/<ID>/entity/<ID>/remove'.
Annotations: allow deletion of duplicate annotations

If an annotations has been linked multiple times to the same entity, one should
be able to delete it. This wasn't possible before and this commit fixes that. Of
course, linking an annotation to the same entity multiple times shouldn't happen
in the first place.