Bug 33569: Fix sort by relevance in catalog search

Bug 33569: Fix sort by relevance in catalog search

When ordering catalog search by relevance from advanced search, this is not visible in sort order combobox in staff interface.

This is because sort by relevance is a special case not using sort
order.
This patch add a new method C4::Context->default_catalog_sort_by to use
in both staff interface and OPAC.

Test plan :
1.0) On staff interface
1.1) Define system preferences defaultSortField = 'relevance' and defaultSortOrder = 'descending'
1.2) Perform a catalog search from main page on any term, ie 'perl'
=> You see in sort combobox 'relevance' selected (with a check icon)
1.3) Perform an advanced search on any term, ie 'perl', with sort by relevance
=> Without patch : you see in sort combobox 'relevance' not selected (no check icon)
=> With patch : you see in sort combobox 'relevance' selected (with a check icon)
2.0) On OPAC
2.1) Define system preferences OPACdefaultSortField = 'relevance' and OPACdefaultSortOrder = 'descending'
2.2) Perform a catalog search from main page on any term, ie 'perl'
=> You see in sort combobox 'relevance' selected
3.0) Run prove t/Context.t

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>