Add html5-progress-polyfill as a submodule

The spinner on the download grades step will be replaced with a progress element. The HTML5 progress polyfill will be used to cover Internet Explorer 8 and 9, which don't natively support the progress element.
Add progress element to "Download grades" step

Grades download can take a long time for courses with a large number of students. The existing spinner gives insufficient feedback for long operations. A progress element has been inserted and html5-progress-polyfill imported to support IE 8 and 9. Actual progress is not being displayed yet, pending changes in the javascript code.