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.

script/fixes/064_initialize_bulk_update_request_tags.rb

@@ -0,0 +1,11 @@
+#!/usr/bin/env ruby
+
+require_relative "../../config/environment"
+
+BulkUpdateRequest.transaction do
+  BulkUpdateRequest.find_each do |request|
+    request.tags = AliasAndImplicationImporter.new(request.script, nil).affected_tags
+    request.save!(validate: false)
+    puts "bur id=#{request.id} tags=#{request.tags}"
+  end
+end