moodle_page: MDL-12212 deprecate more of the legacy page machinery, and avoid overwriting $PAGE ever

Start calling $PAGE->set_url in all the places it will be necessary
Start of a stub implementation of $PAGE->blocks to stop other things breaking
Remove some of the special case methods in admin_page

moving blocks will now be broken until the new bockslib is done

If ->allowcategorythemes is set, includes all parent categories too

pagetype