robots.txt: block Googlebot from crawling certain useless URLs.

Block Googlebot from crawling certain slow useless URLs. Sometimes
Googlebot tries to crawl old source:<url>, approver:<name>, and
ordfav:<name> searches in bulk, which tends to slow down the site because
things like source:<url> are inherently slow, and because Google spends
hours at a time crawling them in parallel. This is despite the fact that
these links are already marked as nofollow and noindex, and source:<url>
links were removed from posts a long time ago to try to stop Google from
crawling them.

app/views/robots/index.text.erb

@@ -5,6 +5,9 @@ Allow: /$
 <% if !Rails.env.production? || Danbooru.config.hostname == request.host %>
 Disallow: /*.atom
 Disallow: /*.json
+Disallow: /posts?tags=source:*
+Disallow: /posts?tags=ordfav:*
+Disallow: /posts?tags=approver:*
 
 Disallow: <%= artist_urls_path %>
 Disallow: <%= artist_versions_path %>