MDL-29866 page setup improvements

MDL-29866 page setup improvements

incorrect PAGE init - it should be done at the very end; redirect() should not use OUTPUT before PAGE init; SITEID should be deprecated in favour of $SITE->id (this is going to cause troubles in tenant switching in CLI, cron and tests); missing "global $SITE"'; minor coding style issues; PHPDocs; it also helps with merging/testing of multitenant patch