Hold Groups: add patron to a group from within the Patron staff client interface

[Andrea Neiman]

Wishlist - follow up from Hold Groups (bug 1838995)

Currently, Patrons can be removed from hold groups via the Patron --> Hold Groups interface in the web staff client, and added to hold groups from the Patron Search interface.

This wishlist item is to embed a mechanism within the Patron --> Hold Groups interface to search available hold groups and add the patron to the groups.