MDL-42619 block_community: delete course redirect bug fix

When deleting a course HTTP_REFERER is used to determine the return
point for the continue button.
If the user arrives from a page that had recieved required variables
by post then they would be lost as they're not passed by referer
and the user would end up with an error.
I've fixed the course/view.php situation but really the best
solution would be to stop using the referer and instead pass the
URL we want to return to.