CRM-14996 - Support multi-valued fields for state/country chain-select

CRM-14996 - Support multi-valued fields for state/country chain-select

When selecting more than one, it groups the results into option groups for better usability.
Applies select2 to all chain-select fields to avoid any push-me/pull-me in future patches to the js
(e.g. fixing the code to work for normal selects and breaking select2 and vice-versa).