Make it possible to select calendar as node

[tobbexiv]

This makes it possible to easily add a calendar page directly in the site edit menu. Also this makes it possible to really overwrite the slug and not only create aliases. Otherwise you are forced to use the link option, which is not self explanatory and in case of category selection doe not display the alias :)

Configuration for the blog extension: https://github.com/pagekit/extension-blog/blob/master/index.php#L17