posts/index: fix several "This tag is under discussion" issues.

Several fixes for the "This tag is under discussion" notice on the post
index page:

* Fix the notice appearing for BURs that aren't pending.
* Fix the notice never going away because of the cache never expiring.
* List all topics when a tag is involved in multiple BURs.
* Link to the forum post instead of the forum topic (fix #4421).
* Optimization: don't check for BURs when the search isn't a simple
  single tag search.
* Add a `tags` field to the bulk update requests table for tracking all
  tags involved in the request (excluding tags in mass updates that are
  negated/optional/wildcards). Known issue: doesn't handle tag type
  prefixes in mass updates correctly (e.g. `mass update foo -> artist:bar`
  doesn't detect the tag `bar`).
* Allow searching the /bulk_update_requests page by tags.

We don't really need to cache the notice here, but we do it anyway to
reduce queries on the post index page.

test/functional/posts_controller_test.rb

@@ -70,6 +70,12 @@ class PostsControllerTest < ActionDispatch::IntegrationTest
           get posts_path(tags: "search:all")
           assert_response :success
         end
+
+        should "show a notice for a single tag search with a pending BUR" do
+          create(:bulk_update_request, script: "create alias foo -> bar")
+          get_auth posts_path(tags: "foo"), @user
+          assert_select ".tag-change-notice"
+        end
       end
 
       context "with a multi-tag search" do