wiki_pages/show: fix <meta> description tag.

Use an excerpt of the first paragraph rather than the entire page.

app/helpers/wiki_pages_helper.rb

@@ -1,4 +1,9 @@
 module WikiPagesHelper
+  def wiki_page_excerpt(wiki_page)
+    text = strip_dtext(wiki_page.body).split(/\r\n|\r|\n/).first
+    truncate(text, length: 160)
+  end
+
   def wiki_page_other_names_list(wiki_page)
     names_html = wiki_page.other_names.map {|name| link_to(name, "http://www.pixiv.net/search.php?s_mode=s_tag_full&word=#{u(name)}", :class => "wiki-other-name")}
     names_html.join(" ").html_safe

app/presenters/wiki_page_presenter.rb

@@ -5,10 +5,6 @@ class WikiPagePresenter
     @wiki_page = wiki_page
   end
 
-  def excerpt
-    wiki_page.body
-  end
-
   # Produce a formatted page that shows the difference between two versions of a page.
   def diff(other_version)
     pattern = Regexp.new('(?:<.+?>)|(?:[0-9_A-Za-z\x80-\xff]+[\x09\x20]?)|(?:[ \t]+)|(?:\r?\n)|(?:.+?)')

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

@@ -28,7 +28,7 @@
           <p><%= wiki_page_other_names_list(wiki_page) %></p>
         <% end %>
 
-        <%= format_text(wiki_page.presenter.excerpt) %>
+        <%= format_text(wiki_page.body) %>
         <%= render "tag_relationships/alias_and_implication_list", tag: wiki_page.tag %>
 
         <p class="links">

app/views/wiki_pages/show.html.erb

@@ -43,7 +43,7 @@
 <% end %>
 
 <% content_for(:html_header) do %>
-  <meta name="description" content="<%= strip_dtext(@wiki_page.presenter.excerpt) %>"></meta>
+  <%= tag.meta name: "description", content: wiki_page_excerpt(@wiki_page) %>
 <% end %>
 
 <%= render "secondary_links" %>