Bug 12478: allow more granular sorting configuration

Bug 12478: allow more granular sorting configuration

This allows sorting to be configured within a field. For example, while
many values are included for search on author, sorting should only be
done on the main entry values. This permits that by have a sort value,
which can be true, false, or null. true and null are pretty much the
same, but false means that a field isn't available for sorting on. By
default (null), fields can be sorted on.

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jesse Weaver <jweaver@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>