Connectivity widget: fix partner filtering

Filtering in connectivity widget tables was using data from all columns.
Because synapse count columns' HTML title contains the names of the core
selected neurons, any filter term involving these names would return all
rows.  While this should be correctable with DataTables' ofnSearch
function, like oSort for sorting values, ofnSearch is broken in
DataTables 1.9.4.

Instead, only set the partner neuron name column as searchable. While
this removes the ability to search on other columns, given their data
this is an unlikely use case and the filter field is already titled
"Filter partners".

Closes acardona/CATMAID#784.