Fix #4275: Unable to update "banned artist" entries.

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.

app/logical/post_sets/post.rb

@@ -26,7 +26,7 @@ module PostSets
 
     def wiki_page
       return nil unless tag.present? && tag.wiki_page.present?
-      return nil unless !tag.wiki_page.is_deleted? && tag.wiki_page.visible?
+      return nil unless !tag.wiki_page.is_deleted?
       tag.wiki_page
     end
 
@@ -37,7 +37,7 @@ module PostSets
 
     def artist
       return nil unless tag.present? && tag.category == Tag.categories.artist
-      return nil unless tag.artist.present? && tag.artist.is_active? && tag.artist.visible?
+      return nil unless tag.artist.present? && tag.artist.is_active?
       tag.artist
     end