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.
* 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.
