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.