Hide banned artist pages from anonymous (logged out) users. Partial
revert of 6b066f2ca. Artists sometimes ask us to remove all their
information from the site, including their artist profile. It's easier
to hide their profile than to explain to Japanese artists that a) we're
under no obligation to remove their profile and b) we need to keep it to
maintain the ban on their content that they asked for.
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.
Avoid some queries used in wiki page excerpts:
* Only try to load the artist if the tag is an artist tag.
* Avoid using `exists?` queries for wiki pages.
* Bugfix: don't show wiki excerpts for deleted wikis.
