LP#1939154: allow clearing default pickup lib in AngularJS patron editor

LP#1939154: allow clearing default pickup lib in AngularJS patron editor

This patch adds a "Clear Default Hold Pickup Location" button
to enable removing that user-setting. This works around an issue
with the AngularJS OU selector where it doesn't pass an event
to signify to calling code that the user has de-selected any OUs.

To test
-------
[1] Apply the patch.
[2] Use the AngularJS editor to add and clear the default
    pickup library for a patron.
[3] Verify that the changes are saved and displayed as
    expected.

Release-note: Enable clearing the default pickup location in the patron editor.

Signed-off-by: Galen Charlton <gmc@equinoxOLI.org>
Signed-off-by: Jane Sandberg <sandbergja@gmail.com>
Signed-off-by: Jason Boyer <JBoyer@equinoxOLI.org>