T/search 2.1

A couple of changes for the upcoming 2.1 release. The most notable:

 - the search service does not register itself in the central service manager, it is defined as a invokable in the service manager configuration
 - skip CitationTest if short_open_tags is disabled
 - the event manager namespace 'VuFind\Search' should be deprecated in 2.2; maybe even change the ns to 'VuFindSearch\Service', i.e. the class name