Refactor export progress bar to get rid of iframe

[Robert Lyon]

On the 'export your portfolio' page the progress bar does not show in 15.10

This needs to be fixed up - and it would be good to fix it so that it works like the admin bulk processes - see patch https://reviews.mahara.org/#/c/3624/21

I've made this medium priority as the export still works even if we don't see the progress bar - but it still is a regression

[Robert Lyon (robertl-9)]

By the looks of things the changes to admin/bulkexport.php is what we need to emulate for export/index.php

So instead of having the export/download.php file being called in iframe we have it's processes be part of export/index.php and rely on using the new process bar

[Kristina Hoeppner (kris-hoeppner)]

Jen fixed the progress bar in https://reviews.mahara.org/#/c/5326/

It should be reworked though to get rid of the iframe.