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/models/artist.rb

@@ -538,8 +538,4 @@ class Artist < ApplicationRecord
       "Deleted"
     end
   end
-
-  def visible?
-    !is_banned? || CurrentUser.is_gold?
-  end
 end

app/models/artist_version.rb

@@ -4,7 +4,6 @@ class ArtistVersion < ApplicationRecord
 
   belongs_to_updater
   belongs_to :artist
-  delegate :visible?, :to => :artist
 
   module SearchMethods
     def search(params)

app/models/wiki_page.rb

@@ -244,10 +244,6 @@ class WikiPage < ApplicationRecord
     TagAlias.to_aliased(titles & tags)
   end
 
-  def visible?
-    artist.blank? || !artist.is_banned? || CurrentUser.is_builder?
-  end
-
   def to_param
     if title =~ /\A\d+\z/
       "~#{title}"

app/models/wiki_page_version.rb

@@ -3,7 +3,6 @@ class WikiPageVersion < ApplicationRecord
   belongs_to :wiki_page
   belongs_to_updater
   belongs_to :artist, optional: true
-  delegate :visible?, :to => :wiki_page
 
   module SearchMethods
     def search(params)