DEV: Support comma seperated value in order filter for /filter route (#21318)

This allows multiple ordering to be specified by using a comma seperated string.
For example, `order:created,views` would order the topics by
`Topic#created_at` and then `Topic#views.

lib/topics_filter.rb

@@ -132,6 +132,8 @@ class TopicsFilter
          "posters-min", "posters-max", "views-min", "views-max"
       value = values.last
       value if value =~ /\A\d+\z/
+    when "order"
+      values.flat_map { |value| value.split(",") }
     when "created-by"
       values.flat_map { |value| value.split(",").map { |username| username.delete_prefix("@") } }
     else