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.

db/structure.sql

@@ -669,7 +669,8 @@ CREATE TABLE public.bulk_update_requests (
     created_at timestamp without time zone NOT NULL,
     updated_at timestamp without time zone NOT NULL,
     approver_id integer,
-    forum_post_id integer
+    forum_post_id integer,
+    tags text[] DEFAULT '{}'::text[] NOT NULL
 );
 
 
@@ -4732,6 +4733,13 @@ CREATE INDEX index_bans_on_user_id ON public.bans USING btree (user_id);
 CREATE INDEX index_bulk_update_requests_on_forum_post_id ON public.bulk_update_requests USING btree (forum_post_id);
 
 
+--
+-- Name: index_bulk_update_requests_on_tags; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE INDEX index_bulk_update_requests_on_tags ON public.bulk_update_requests USING gin (tags);
+
+
 --
 -- Name: index_comment_votes_on_comment_id; Type: INDEX; Schema: public; Owner: -
 --
@@ -7384,6 +7392,7 @@ INSERT INTO "schema_migrations" (version) VALUES
 ('20200325073456'),
 ('20200325074859'),
 ('20200403210353'),
-('20200406054838');
+('20200406054838'),
+('20200427190519');