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/generate.html

@@ -1,7 +1,7 @@
 {% extends "templates/base.html" %}
 
 {% block content %}
-<a href="/category/{{ blog.id }}">Categories</a>
+<a href="/blog/{{ blog.id }}">Home</a> <a href="/category/{{ blog.id }}">Categories</a> <a href="/tag/{{ blog.id }}">Tags</a>
 {% if user.is_superuser %}
 <form action="/blog/edit/{{ blog.id }}" method="post">
 {% csrf_token %}