Merge (move) 'abstractpages' between normal 'pages' and 'xpages'

In CMS-13916 the hst:xpages children have been merged into hst:pages.

Now we should do the same for 'hst:abstractpages'.

To still be able to identify what an abstract page is and what a concrete page is, we will have to introduce next to nodetype 'hst:xpage' also

hst:abstractpage
hst:page

and make sure the types are 'abstract' and 'page'