Additional settings for schemas

[hunvreus]

We need to add a few more settings for schemas;

• Default extension; .md, .html or anything else you may need.
• Description; it'd be nice to have a description for the content type or form so that we can describe what it is and display it in the UI to help the user understand what is what.
• Collection/Singleton; that's related to #55, we'll need to define if it's a single file or a collection.

[JuhaS]

For Default extension and Description we'd need to add new root-level fields to the schema:

Proposal (see 2nd and 3rd fields that are new):

{ 
  "title": "Pages",
  "description": "The pages for the website",
  "extension": "md",
  "jekyll": {
    "type": "content",
    "id": "posts"
  },
  "JSONSchema": {
    "type": "object",
    "required": [
      "body"
    ],
    "properties": {
      "layout": {
        "type": "string",
        "title": "Layout"
      },
      "title": {
        "type": "string",
        "title": "Title"
      },
      "permalink": {
        "type": "string",
        "title": "Permalink"
      },
      "body": {
        "type": "string",
        "title": "Body"
      }
    }
  },
  "uiSchema": {
    "body": {
      "ui:widget": "customCodeMirror"      
    }
  }
}

We would then read this on the frontend and:

• Show the description at the top of the create/edit form
• Use the extension when creating new field.

If that's ok I can create separate implementation issue for that.

About the Collection/Singleton we can discuss in #55 about that.