Removed thank-you-wrapper

It is now unnecessary since it is a separate page
Made download start on the thank-you page

This allows the back button to function properly.

It also has the benefit of allowing a user to pass a direct link to a
friend, removing the need for someone to even go through the drop
downs. This setup is a good balance between a direct link (where there
is no information provided) and the drop-downs (which a friend may have
already filled out when linking).
Changed width and height for the iframe to 0
Changed line to keep bracket consistency
Added functionality to prevent duplicate downloads

Added checks to ensure that another download is not started when the
back button is pressed.

This works by storing a value in a form, which is saved and then
checked once the back button is pressed. If the user chooses to reload
the page (which would be an intentional action) or navigates back to
the main download page and presses the download button again, then the
download still works.