Navigator: don't show annotation usage count in neuron node

The neuron node displays a list of annotations linked to the neuron
currently displayed. Since annotations can only be linked once to one
entity, there is not much sense to display the usage count for each
annotation---it would always be one.
Remove unused variable
Annotations: query sub-annotation IDs only if requested

This commit makes sure that the list of sub-annotation IDs for a set of
annotations is only retrieved if these are actually requested. This
saves one query in a standard search.
Annotations: fix small style error in basic query generation

Although Django is clever enough to know that there is no join needed when we
filter model objects on a foreign key field by using "field__id" instead of
"field_id", it is clearer and more readable to use the latter.