Add loading icon to XML regenerate previews button
Add the Loading icon to the admin review screen "regenerate previews" button, while previews are generating.
There is a (sometimes significant) delay for the PDF preview, talk to @nmarinos about ways to query whether the PDF preview is done from the front end. It may require adding a column to the manuscript graphql query response for this page.
The PDF conversion will only work on the beta site, not locally or on staging. So for ease of testing, please make sure the loading stops if the PDF conversion is successful, OR if a user uploads a PDF preview file ('pdf4print' type) manually.
The loading icon should still be there if the user leaves the page and returns while the conversions are still processing.