Dashboard models

This PR depends on the django-cms PR
We are developing for the future. What if the name of the community has spaces or special characters? Are we going to use underscores or hyphens instead of spaces? How are the special characters going to be substituted in the URL?

Even if we decide to use simply the name of the community to generate the resource area, the absolute URL doesn't seem to be a flexible solution. So maybe use the SlugField?

We didn't talk over the URL scheme that we want to use, so I have create a [public pad](https://etherpad.mozilla.org/systers-portal) for us discuss it.