MDL-32652 - Make block drag-drop work throughout Moodle, part 3

The call to init_requirements_data (which includes the block drag-drop JS)
happens at a late enough stage in page generation that we can simply
pass the context ID through to the JS, so that it can be sent as part of
the AJAX request.  This prevents lib/ajax/blocks.php from having to try
to guess the context from the other bits of information it receives, so
block drag-drop should now work everywhere.