Introduces secondary sort by date

Why are these changes being introduced:

* spatial searches with no other arguments do not sort by relevance
  because all results are equal 1.0 score matches
* introducing a secondary sort by date desc will allow more consistent
  and predictable serach result ordering (previously local and deployed
  indexes were not seeing the same result ordering)

Relevant ticket(s):

* https://mitlibraries.atlassian.net/browse/GDT-185

How does this address that need:

* explicitly adds the sorting by score as the primary sort. This is the
  default behavior, but we need to introduce it explicitly in order to
  add the secondary sort
* adds secondary sort by `dates.value.as_date` descending

Document any side effects to this change:

* There should be none as any scored results will still take precedent.
  This has been confirmed by running the same searches locally against
  dev1 with this change and comparing them the results prior to this
  change

Author: JPrevost

app/models/opensearch.rb

@@ -20,7 +20,8 @@ def build_query(from)
       from:,
       size: SIZE,
       query:,
-      aggregations:
+      aggregations:,
+      sort:
     }
 
     query_hash[:highlight] = highlight if @highlight
@@ -39,6 +40,20 @@ def query
     }
   end
 
+  def sort
+    [
+      { _score: { order: 'desc' } },
+      {
+        'dates.value.as_date': {
+          order: 'desc',
+          nested: {
+            path: 'dates'
+          }
+        }
+      }
+    ]
+  end
+
   def highlight
     {
       pre_tags: [