DLSR-285: change preferred title sort field to sort_title_ssort

[sourcefilter]

Leaves a fallback sort by title_alpha_numeric_ssort, so current functionality will remain until solr config is updated and index is reindexed.

Sorting by exists(field_name) asc before sorting by the field itself ensures that records where the field is empty fall to the back. (Otherwise, they would come first in the descending search).

[akohler]

@sourcefilter Makes sense, approved. Checks are still running so I'll leave this for you to merge & deploy when all is ready.
Thanks --Andy

[akohler]

@sourcefilter I see the rspec check failed, in a way related to this change. Probably the test should be updated?

Failures:

  1) CatalogController sort fields has exactly expected sort fields
     Failure/Error: expect(sort_fields).to contain_exactly(*expected_sort_fields)

       expected collection contained:  ["date_dtsort asc", "date_dtsort desc", "score desc", "title_alpha_numeric_ssort asc", "title_alpha_numeric_ssort desc"]
       actual collection contained:    ["date_dtsort asc", "date_dtsort desc", "exists(sort_title_ssort) asc, sort_title_ssort  desc, exists...itle_ssort asc, exists(title_alpha_numeric_ssort) asc, title_alpha_numeric_ssort asc", "score desc"]
       the missing elements were:      ["title_alpha_numeric_ssort asc", "title_alpha_numeric_ssort desc"]
       the extra elements were:        ["exists(sort_title_ssort) asc, sort_title_ssort  desc, exists(title_alpha_numeric_ssort) asc, title_...t) asc, sort_title_ssort asc, exists(title_alpha_numeric_ssort) asc, title_alpha_numeric_ssort asc"]
     # ./spec/controllers/catalog_controller_spec.rb:237:in `block (3 levels) in <top (required)>'
     # /usr/local/bundle/ruby/2.7.0/gems/webmock-3.14.0/lib/webmock/rspec.rb:37:in `block (2 levels) in <top (required)>'

[akohler]

All checks are passing now, feel free to merge & deploy. Thanks --Andy