FEATURE: Add setting & preference for search sort default order (#24428)

This commit adds a new `search_default_sort_order` site setting,
set to "relevance" by default, that controls the default sort order
for the full page /search route.

If the user changes the order in the dropdown on that page, we remember
their preference automatically, and it takes precedence over the site
setting as a default from then on. This way people who prefer e.g.
Latest Post as their default can make it so.

config/site_settings.yml

@@ -2389,6 +2389,11 @@ search:
   search_page_size:
     default: 50
     hidden: true
+  search_default_sort_order:
+    default: 0 # "relevance"
+    client: true
+    type: enum
+    enum: "SearchSortOrderSiteSetting"
 
 uncategorized:
   version_checks: