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.

app/logical/alias_and_implication_importer.rb

@@ -79,27 +79,17 @@ class AliasAndImplicationImporter
   end
 
   def affected_tags
-    tokens = self.class.tokenize(text)
-    tokens.inject([]) do |all, token|
-      case token[0]
+    AliasAndImplicationImporter.tokenize(text).flat_map do |type, *args|
+      case type
       when :create_alias, :remove_alias, :create_implication, :remove_implication
-        all << token[1]
-        all << token[2]
-        all
-
+        [args[0], args[1]]
       when :mass_update
-        all += PostQueryBuilder.new(token[1]).split_query
-        all += PostQueryBuilder.new(token[2]).parse_tag_edit
-        all
-
+        tags = PostQueryBuilder.new(args[0]).tags + PostQueryBuilder.new(args[1]).tags
+        tags.reject(&:negated).reject(&:optional).reject(&:wildcard).map(&:name)
       when :change_category
-        all << token[1]
-        all
-
-      else
-        all
+        args[0]
       end
-    end
+    end.sort.uniq
   end
 
   private

app/logical/post_sets/post.rb

@@ -131,5 +131,10 @@ module PostSets
       # be smarter about this in the future
       posts.reject(&:is_deleted).select(&:visible?).max_by(&:fav_count)
     end
+
+    def pending_bulk_update_requests
+      return BulkUpdateRequest.none unless query.is_simple_tag?
+      @pending_bulk_update_requests ||= BulkUpdateRequest.pending.where_array_includes_any(:tags, query.tags.first.name)
+    end
   end
 end

app/logical/tag_change_notice_service.rb

@@ -1,13 +0,0 @@
-module TagChangeNoticeService
-  module_function
-
-  def get_forum_topic_id(tag)
-    Cache.get("tcn:#{tag}")
-  end
-
-  def update_cache(affected_tags, forum_topic_id)
-    affected_tags.each do |tag|
-      Cache.put("tcn:#{tag}", forum_topic_id, 1.week)
-    end
-  end
-end