Better support 'link not found' in PageModelAPI

Below improvement can be done for PMA v0.9 and 1.0

Currently, we have links something like:

"links" : {
            "site" : {
              "href" : "/news",
              "type" : "internal"
            }
          }

and in html content something like"

<a href=\"/news\" data-type=\"internal\">news</a>

the type (data-type) can be one of these at the moment:

    public enum LinkType {

        RESOURCE("resource"),
        EXTERNAL("external"),
        INTERNAL("internal");

However, a link can be to a document for which NO URL can be created in the HST sitemap OR a link can be to a deleted or not live document. In that case, we have a broken link. We need to include a type for that.

After discussing with mdokolin, we decided that the SPA has enough info if for a unresolvable link we serialize it as:

"links" : {
            "site" : {
              "type" : "unknown"
            }
          }

or

<a data-type=\"unknown\">news</a>