Fixes external blog posts in search (alshedivat#2470)

Fixes alshedivat#2469. Separated `news` and `posts` from other collections in
search, since it caused duplicated entries. Also to ensure they are in
chronological reverse order.

Signed-off-by: George Araujo <george.gcac@gmail.com>

_layouts/post.liquid

@@ -62,12 +62,6 @@ layout: default
   </header>
 
   <article class="post-content">
-    {% if page.toc and page.toc.beginning %}
-      <div id="table-of-contents">
-        {% toc %}
-      </div>
-      <hr>
-    {% endif %}
     <div id="markdown-content">
       {{ content }}
     </div>