artists: prevent Google from indexing banned artists.

Prevent search engines from indexing artist pages, wiki pages, and tag
searches for banned artists.

app/views/artists/show.html.erb

@@ -1,3 +1,5 @@
+<% noindex if @artist.is_banned? %>
+
 <%= render layout: "show" do %>
   <div>
     <%= render "summary", artist: @artist %>

app/views/posts/partials/index/_seo_meta_tags.html.erb

@@ -14,9 +14,7 @@
   <% canonical_url root_url(host: Danbooru.config.hostname) %>
 <% end %>
 
-<% if @post_set.hide_from_crawler? %>
-  <meta name="robots" content="nofollow,noindex">
-<% end %>
+<% noindex if @post_set.hide_from_crawler? %>
 
 <% if @post_set.has_explicit? %>
   <meta name="rating" content="adult">

app/views/wiki_pages/show.html.erb

@@ -1,6 +1,10 @@
 <% page_title "#{@wiki_page.pretty_title.titleize} Wiki" %>
 <% meta_description DText.excerpt(@wiki_page.body) %>
 
+<% if @wiki_page.artist.present? && @wiki_page.artist.is_banned? %>
+  <% noindex %>
+<% end %>
+
 <%= render "secondary_links" %>
 <%= render "sidebar" %>