Dashboard: Create djangoldp-dashboard model

Create a djangoldp package djangoldp-dashboard which will serve this:

{
  "@id":"http://server/dashboards/",
  "@type":"ldp:Container",
  "ldp:contains":[
    {
      "@id": "http://server/dashboards/1/",
      "size": "1",
      "content": "Coucou.",
      "permissions":[{"mode":{"@type":"view"}}]
    }
  ],
  "@context":"https://cdn.happy-dev.fr/owl/hdcontext.jsonld"
}

size is a number (1, 2 or 3) defining the size of the box.

content is a simple text.

https://git.startinblox.com/djangoldp-packages/djangoldp-dashboard