autocomplete: fix labels for static metatags.

When completing a static metatag, such as rating: or order:,
don't include the metatag name in the autocomplete menu.

For example, when completing `rating:g`, show `general` in the
autocomplete menu, not `rating:general`.

This makes static metatags consistent with other metatags.

app/logical/autocomplete_service.rb

@@ -270,7 +270,7 @@ class AutocompleteService
     results = values.select { |v| v.starts_with?(value.downcase) }.sort.take(limit)
 
     results.map do |v|
-      { label: metatag + ":" + v, value: v }
+      { type: "static", label: v, value: v }
     end
   end