Add tags support

Set slugify to be global
Update modification date only if text has been changed
Fix a bug in preview (post_id was passed instead of blog_id)
Add Home, Category and Tag links in blog area instead of global area (dynastie template)
Fix a bug in Have_I_right

templates/category.html

@@ -1,7 +1,6 @@
 {% extends "templates/base.html" %}
 
 {% block content %}
-<table>
 {% if categories|length == 0 %}
 <b>Any category available</b><br/><br/>
 {% else %}
@@ -12,5 +11,4 @@
 </table>
 {% endif %}
 <li><a href="/category/add">Add a category</a></li>
-</table>
 {% endblock %}