LP#1756110: fix egBasicComboBox drop-down functionality

LP#1756110: fix egBasicComboBox drop-down functionality

Somehow egBasicComboBox got missed when it became necessary to
switch to the newer way of specifying Bootstrap drop-downs. This
patch rectifies that and fixes the drop-down functionality.

To test
-------
[1] Create a report template in the web staff report editor and
    add a filter field.
[2] Attempt to change the filter operator. Note that nothing happens
    when the drop-down button is clicked.
[3] Apply the patch and repeat step 2. This time, the drop-down
    should work and display the list of available filter operators.

Other places affected include:

- The part selector in the volume/copy editor
- Template selector in the volume/copy editor
- Template selector in the labels interface
- Routing label selector in the subscription manager
- Call number prefix and suffix selectors in the serials batch receiving
  interface

Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>