See issue #133

Add status attribute to classes

Inform Max and Yuki about which classes can be attached to resources using the "status" attribute.
A value of 1 indicates that it can be attached to resources (and a value of 0 disallows this behavior)
Thus, classes.json will now return

{
    "name":"Class 1",
    "author_name":"Alena Kiehn",
    "status":1,
    "created":"Jan 1, 2019 11:53 PM",
    "updated":"Jan 1, 2019 11:54 PM"
}

and nodes returned by tree.class will also have a status attribute.

Make class_id required for resource creation

Make class_id a required attribute to be sent in resources.store. This is what it should look like from now on:

{
    "name":"Resource 2",
    "use_id":1,
    "class_id":1,
    "contents":[
        {
            "name":"Resource Content 2",
            "type":"link",
            "content":"http://www.hessel.com"
        }
    ]
}

where class_id can be 0 if you want to attach the resource to the root.