Neuron name service: handle bad unregistrations

Always treat `skids` as an array of skeleton IDs in `unregister`
(unlike `managedSkeletons`, which uses IDs as object keys), and do
nothing for requests to unregister a skeleton ID which was not
previously registered.
Fix neuron name display in canvas topbar

A previous commit broke the active neuron name being displayed in the
topbar when selecting a node (a115b9808b1443280f667a44be204f1945fb31ef).
This fixes that regression and updates neuron name display in the topbar
to use the neuron name service. Manual updates of the topbar made
redundant by this change are removed, including `updateNeuronNameLabel`.

Closes acardona/CATMAID#738.