Fix #5057: Modqueue: filtering by tag breaks ordering

Fix the order dropdown box on the modqueue page not working when filtering by tag.

This happened because when you do a tag search, the default order is set to `ORDER BY posts.id DESC`.
When you applied another order with the dropdown box, the new order would be tacked on to the old
ordering as a tiebreaker instead of replacing it, producing e.g. `ORDER BY posts.id DESC, queued_at DESC`
instead of `ORDER BY queued_at DESC`. The default order would always win because `posts.id` is
unique and doesn't have ties.

The fix is to have orders always override the previous order instead of adding to it.

Note that now if you use an `order:`, `ordfav:`, or `ordpool:` metatag in the search box on the
modqueue page, they will always be ignored in favor of the dropdown box.

test/functional/modqueue_controller_test.rb

@@ -26,6 +26,17 @@ class ModqueueControllerTest < ActionDispatch::IntegrationTest
         assert_equal([{ "rating" => @post.rating }], response.parsed_body)
       end
 
+      should "order posts correctly when searching for tags" do
+        post1 = create(:post, tag_string: "touhou", is_pending: true, score: 5)
+        post2 = create(:post, tag_string: "touhou", is_pending: true, score: 10)
+        post3 = create(:post, tag_string: "touhou", is_pending: true, score: 15)
+
+        get_auth modqueue_index_path(search: { tags: "touhou", order: "score_asc" }), @admin, as: :json
+
+        assert_response :success
+        assert_equal([post1.id, post2.id, post3.id], response.parsed_body.pluck("id"))
+      end
+
       should "include appealed posts in the modqueue" do
         @appeal = create(:post_appeal)
         get_auth modqueue_index_path, @admin