pools: stop using the pool_string field (#4160).

Stop using the pool_string field internally, but keep maintaining it
until we can drop it later.

* Stop using the pool_string for `pool:<name>` metatag searches.
* Stop using the pool_string in the `Post#pools` method. This is used to
  get the list of pools on post show pages.

test/functional/posts_controller_test.rb

@@ -143,6 +143,16 @@ class PostsControllerTest < ActionDispatch::IntegrationTest
         assert_response :success
       end
 
+      context "with pools" do
+        should "render the pool list" do
+          as(@user) { @post.update(tag_string: "newpool:comic") }
+          get post_path(@post)
+
+          assert_response :success
+          assert_select "#pool-nav .pool-name", /Pool: comic/
+        end
+      end
+
       context "with only deleted comments" do
         setup do
           as(@user) { create(:comment, post: @post, is_deleted: true) }