Add possibility to use the Single Pid from a news category

This change introduces the GET-Param &useCategorySinglePid
which makes the news urls use the Page Ids from the first category
of each news item. It falls back to the Pid specified with &singlePid.

This is relevant for the following setup:
* All tt_news records are in a single folder (so splitting the up into several sitemaps isn't possible)
* There are several single view pages referenced by the different news catgories