* Deprecated tags can't be added to posts, but existing deprecated tags
in a post won't be removed
* Only empty tags can be marked as deprecated manually
* No tags can be manually undeprecated
** These limits don't apply to admins
* Deprecating or undeprecating a tag will create a new mod action to
prevent people from going rogue
* Added deprecate/undeprecate commands for BURs
* Deprecating a tag via BUR removes all implications to and from it as well
Add rel=nofollow to various internal search links to prevent Google from
attempting to crawl things like `<tag> status:deleted` or `approver:<name>`
searches.
* Replace the .category-N CSS classes on tags with .tag-type-N. Before
we were inconsistent about whether tag colors were indicated with
.category-N or .tag-type-N. Now it's always .tag-type-N.
* Fix various places to not use Tag.category_for. Tag.category_for does
one Redis call per tag lookup, which leads to N Redis calls on many
pages. This was inefficient because usually we either already had the
tags from the database, or we could fetch them easily.
Allow all users to view and edit artist entries and wiki pages belonging
to banned artists. There was little need to hide these pages from
Members, it was mainly to appease artists who didn't like us even
linking to their sites.
These restrictions also had multiple flaws:
* Banned artist information was still visible in the API.
* It was still possible to edit banned artists using the API.
* It was still possible for unprivileged users to revert banned
artist entries or wiki pages to previous versions.
* The restrictions were inconsistent: in various places they were
either Member-only, Gold-only, or Builder-only.
* Redirect the show_or_new action to either the show page or the new
page. Don't use show_or_new to render nonexistent wikis; do that in the
regular show action instead.
* Make the show action return 404 for nonexistent wikis.
* Factor out common sidebar layout template.
* Convert wiki pages and posts to use this template.
* Add data-layout attribute to <body> element indicating the current layout.
