Wait until saving has finished before resetting PDFViewerApplication._saveInProgress (PR 12137 follow-up)

I obviously missed this during review, but currently `PDFViewerApplication._saveInProgress` is reset *synchronously* in `PDFViewerApplication.save`.
That was probably not intended, since it essentially renders the `PDFViewerApplication._saveInProgress` check pointless given that the actual saving is an *asynchronous* operation.

web/app.js

@@ -939,8 +939,10 @@ const PDFViewerApplication = {
       })
       .catch(() => {
         this.download();
-      }); // Error occurred, try downloading with the URL.
+      })
+      .finally(() => {
         this._saveInProgress = false;
+      });
   },
 
   /**