Neuron navigator: move biggest part of active node code into mixin

By moving the active node code into a mixing, the active node functionality can
be reused in other nodes as well.
Neuron navigator: make home node display the active neuron

Displaying information about the active node is a very common use case for the
neuron navigator. Therefore, this information should be easily accessible. In
contrast, the list of all neurons isn't used that frequently. This commit swaps
both displays. The home node now shows information about the active skeleton and
provides a link to display all neurons.