Add image hover

If a user hovers over an image with their mouse, it will load the image
and display it in the corner.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

board/templates/board/post_snippet.html

@@ -50,7 +50,7 @@ at {{post_created}}
 {# Image thumbnail #}
 <div class="post_image_thumbnail">
     <a href="{{post.image.url}}" target="_blank">
-    <img src="{{post.thumbnail.url}}">
+    <img class="post_image" src="{{post.thumbnail.url}}" data-url="{{post.image.url}}">
     </a>
 </div>
 {% endif %}