Improve the Report Download feature

[smcmurtry]

The Report Download feature currently provides a poor user experience when downloading large reports (more than 50,000 rows). Downloading a report that is 1 million rows takes about 20 minutes and the UX does not inspire confidence that the download will complete successfully.

We can improve the UX of this feature by making it work asynchronously. The user clicks a button to generate the report, and then downloads it from a link once it is done being generated.