Connectivity widget: store content element in variable

This simplifies access to the content, makes it less error prone and more
readable.
Connectivity widget: do threshold filtering client side

There is no real reason to do the synapse count threshold filtering on
the back-end. This commit moves it therefore to the client to make the
requests slightly faster to reply to. This is also in preparation to
have up and downstream thresholds for each individual selected neuron
(issue #611).
Connectivity widget: make neuron list a table

This change is done in preparation of adding upstream and downstream
thresholds per neuron. Having these in a table should look nicer than in
a table.
Connectivity widget: fix small code style issues