New nested level between subpages

Now we are able to group together subpages of similar topic.

I have taken special care of not messing up too much with the database.
Only two new fields are added to the Subpage model so django evolution
can handle that automatically.

./manage.py evolve --hint -x should be enough.
The hints should be two new fields in the subpage table.
Run ./manage.py syncdb first since a new table may be needed
(because of the new ManyToMany field in Subpage).

After that, one can edit the subpages using the admin interface and define
what subpages should be grouped together (I can take care of that as soon
as the changes are included in the production server).