LPS-49937 add KBFolder model

KBArticles may now have two types of parents: folder and articles. Add
also a field to KBArticle to indicate the kind of parent pointed to by
`parentResourcePrimKey`:

  - KBArticle: KBArticleConstants.getClassName()
  - KBFolder: KBFolderConstants.getClassName()

The UpgradeProcess will make sure that all existing articles have the
correct parent classNameId:

  - Root articles (parentResourcePrimKey == 0) will have a folder as a
    parent.
  - Non root articles (parentResourcePrimKey <> 0) have an article
    as a parent.
LPS-49937 SF
LPS-49937 SF
LPS-49937 lang keys