Annotations: fix indentation
Annotations: WIP
Annotations: avoid two LEFT OUTER JOINs by two subqueries

The retrieval of the username of the user who used a particular
annotation most recently is now done with a combination of two
subqueries. Before Django's ORM surprisingly created two LEFT OUTER
JOINs for a reverse foreign key lookup.
Annotation navigator: move handler wiring into to table creation

The purpose of the function to create the neuron table is easier re-use. To make
it even easier to re-use the neuron list table, all required handlers for it to
work are now added there as well.
Annotation navigator: let home node create neuron list directly
Annotation navigator: simplify query for annotated entities

Instead of having multiple queries per entity, three queries are now used in
total. This speeds up requesting entities considerably.
Navigator: properly list the first 25 neurons on start up.
neuron_annotations.py: added note, must remove a costly loop over many
irrelevant parameters.
Fix error in neuron_annotation.py: a QuerySet is expected, not a list.
Fix typos in comment
Navigator: use javascript rather than python style for object keys.
Navigator: fix error retrieving the neuron ID for a new NeuronNode.
Navigator: simplify function NeuronListNode/get_entities
Navigator: fix error in setting the neuron ID for the checkboxes in NeuronListNode.
Navigator: fix error in opening a new AnnotationFilterNode when
double-clicking on a meta-annotation.
Navigator: simpler init block for MetaAnnotationListNode.
Navigator: quick fix for meta-annotations. Needs cleaning up.
Navigator: fixed errors with retrieving the list of selected neurons.

Uses an unlinked NeuronListNode internally in the HomeNode and the AnnotationFilterNode.
Could use some refactoring.
Navigator: NeuronNode must use neuron_name and not name for the SkeletonModel.
Otherwise the name of the SkeletonModel generated for the ActiveNeuronNode
is "Active Neuron".
Navigator: added TODO, there should be no need to load twice the annotations
for the ActiveNeuronNode.