Clean up forums.go

All the really highly redundant stuff was pulled into a helper type.

src/templates/src/forum_post_delete.html

@@ -5,6 +5,7 @@
     <h3 class="mb3">Are you sure you want to delete this post?</h3>
     {{ template "forum_post_standalone.html" .Post }}
     <form action="{{ .SubmitUrl }}" method="POST" class="pv3 flex justify-end">
+        {{ csrftoken .Session }}
         <input type="submit" value="Delete Post">
     </form>
 </div>