When you download a file directly from a Moodle form submit button,
the submit button disables when you click it, but you remain on that
page so we need to re-enable the button.
This commit causes it to re-enable once the file download finishes,
setting a temporary cookie to indicate this to the JavaScript code.
It also adds a method to disable the system on a given form by
setting data-double-submit-protection="off".