forum: fix edit timestamp recording + display

On the preproduction server there are messages that have an edit
timestamp some 20 ns after their creation, for some reason.

app/routes/posts/edit.py

@@ -26,7 +26,7 @@ def edit_post(postid):
 
     p = Post.query.filter_by(id=postid).first_or_404()
 
-    # Check permissions. TODO: Allow guests to edit their posts?
+    # Check permissions
     if not current_user.can_edit_post(p):
         abort(403)
 
@@ -68,6 +68,7 @@ def edit_post(postid):
                 attachments.append((a, file))
                 db.session.add(a)
 
+        comment.touch()
         db.session.add(comment)
 
         if isinstance(p, Topic):

app/templates/widgets/thread.html

@@ -61,7 +61,7 @@
         <td class="message">
             <div class="info">
                 <div>Posté le <a href="{{ request.path }}#{{ c.id }}">{{ c.date_created | dyndate }}</a></div>
-                {% if c.date_created != c.date_modified %}
+                {% if (c.date_modified - c.date_created).seconds > 120 %}
                     <div>Modifié le <a href="{{ request.path }}#{{ c.id }}">{{ c.date_modified | dyndate }}</a></div>
                 {% endif %}
                 {{ post_actions(c) }}