Add loading indicator for slow Requests CSV report

Overview: In Q3 2020, it was discovered that the Requests CSV was not including all requests. mattjlc fixed that bug but discovered that the report was now very slow to generate. We planned to add some simple user feedback to the process but didn't want to hold up the release of UIREQ. So I created this spin off bug just for the user feedback. It shouldn't be a release-blocker but would be realllly nice to get in.

Steps to Reproduce:

1. Log into https://bugfest-honeysuckle.folio.ebsco.com/ as folio/folio
2. Go to Requests and filter by type = pages or some other filter that gives 1,000 results (see UIREQ-538 for bug on result counts)
3. Go to Actions > Export search results to CSV and export

Expected Results: Should have a loading indicator similar to the pick slips report (see attached screencast)

Actual Results: Very slow, no user feedback