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.
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