LP#1978889: display alert type for existing item alerts

LP#1978889: display alert type for existing item alerts

This patch ensures that the type of an item alert is
displayed when managing item alerts in the Angular item
attributes editor.

To test
-------
[1] Add an alert to an item.
[2] From the Angular Holdings View, chose 'Edit Item Alerts'
    for that item. Note that the combobox for the existing
    alert does not display a value.
[3] Open the item in the Item Attributes editor and click
    the Item Alerts button. Again, note that the alert type
    combobox does not display a value.
[4] Apply the patch and repeat steps 2 and 3. This time, the alert
    type should be displayed.

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