HPCC-12075 Allow to select a good landing zone for Upload/download

When the Upload/download File menu is selected, FileSpray.DropZoneFiles
service tries to retrieve landing zones and the files from the first
landing zone. If the dafilesrv is down on that landing zone, after
several retries (a long time wait) for retrieving the files, an
exception is sent out without any landing zone displayed. This fix only
retrieves landing zone list after the Upload/download File menu is
clicked. A user may see and choose any landing zone for file upload and
download.

Signed-off-by: wangkx <kevin.wang@lexisnexis.com>