When a BUR is approved, put it in a `processing` state. After it
successfully finishes processing, put it in the `approved` state. If it
fails processing, put it in the `failed` state.
If approving the BUR fails with a validation error, for example because
the alias already exists or an implication lacks a wiki, then leave the
BUR in the `pending` state. The `failed` state is only for unexpected
errors during processing.
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.
