TPAC: Search Filter Groups

TPAC: Search Filter Groups

Adds support for a new CGI param:

fg:<filter_group_code>=filter_group_entry

Each filter group entry will be mapped to a filter_group_entry()
filter and appended to the current search query.

This includes a new filter selector builder at
parts/filter_group_selector.tt2.

Usage, assuming a filter_group with code "audience":

<span>[% ctx.filter_groups.audience.label %]<span>
<span>
[%
    INCLUDE 'opac/parts/filter_group_selector.tt2'
    filter_group=audience
    none_ok=1
%]
<span>

The set of filter groups available to the TPAC are those defined at the
search org unit and its ancestors.  If multiple filters have the same
code name, the filter owned closest to the context org unit is used.
This allows for global filters with local overrides.

Signed-off-by: Bill Erickson <berick@esilibrary.com>
Signed-off-by: Mike Rylander <mrylander@gmail.com>