search: move misc search parsing helpers to PostQueryBuilder.

* Move various search parser helper methods (`has_metatag?`,
  `is_single_tag?` et al) from PostSets and the Tag model to
  PostQueryBuilder.

* Fix various minor bugs stemming from trying to check if a search query
  contains certain metatags using regexes or other adhoc techniques.

app/views/posts/index.html.erb

@@ -19,7 +19,7 @@
 
   <%= render "posts/partials/index/options" %>
 
-  <%= render "posts/partials/index/related" %>
+  <%= render "posts/partials/index/related", post_set: @post_set %>
 <% end %>
 
 <% content_for(:content) do %>

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

@@ -15,7 +15,7 @@
 
     <li><%= link_to "Deleted", posts_path(tags: "#{params[:tags]} status:deleted") %></li>
     <li><%= link_to "Random", random_posts_path(tags: params[:tags]), id: "random-post", "data-shortcut": "r" %></li>
-    <% if Tag.is_simple_tag?(params[:tags]) %>
+    <% if post_set.query.is_simple_tag? %>
       <li><%= link_to "History", post_versions_path(search: { changed_tags: params[:tags] }) %></li>
     <% end %>
     <li><%= link_to "Count", posts_counts_path(:tags => params[:tags]) %></li>

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

@@ -1,4 +1,4 @@
-<% if @post_set.is_empty_tag? %>
+<% if @post_set.query.is_empty_search? %>
   <% page_title("#{Danbooru.config.app_name}: Anime Image Board", suffix: nil) %>
   <% meta_description("#{Danbooru.config.canonical_app_name} is the original anime image 'booru. Find over 3.75 million anime pictures categorized by over 100 million tags.") %>