views: refactor page titles.

Refactor `page_title` helper to automatically include site name.

app/views/wiki_pages/show.html.erb

@@ -1,3 +1,6 @@
+<% page_title "#{@wiki_page.pretty_title.titleize} Wiki" %>
+
+<%= render "secondary_links" %>
 <%= render "sidebar" %>
 
 <% content_for(:content) do %>
@@ -38,12 +41,6 @@
   <%= render "wiki_pages/posts", wiki_page: @wiki_page %>
 <% end %>
 
-<% content_for(:page_title) do %>
-  Wiki - <%= @wiki_page.pretty_title %> - <%= Danbooru.config.app_name %>
-<% end %>
-
 <% content_for(:html_header) do %>
   <%= tag.meta name: "description", content: wiki_page_excerpt(@wiki_page) %>
 <% end %>
-
-<%= render "secondary_links" %>