Backport 3.1.2 When choosing a folder in the sitemenu item picker, the folder can be incorrectly selectable as link

The problem is as follows:

When for example the sitemap looks as follows:

-news  (contentpath = news)
   **.html  (contentpath = news/${1})

then, the folder /news/2010 resolves via the link rewriting to /news/2010.html. It is debatable whether this is a good thing. We should not change this in the linkrewriting because we risk backwards incompatible changed behavior. Instead in org.hippoecm.hst.pagecomposer.jaxrs.model.treepicker.DocumentTreePickerRepresentation which makes the folder/document representation, I will test whether the created link matches a sitemapitem with extension (eg .html) but the node was a folder: In that case, set selectable = false