DM-8181: Add script download dialog

[loitly]

https://jira.lsstcorp.org/browse/DM-8181

To test:

• launch /firefly/lc.html
• upload a raw table
• click download on top right corner
• enter at least 20 images, so that more than one download parts appear in the background monitor
• when package completes, click on "Get Download Script"
• play around with the different parameters to make sure it's working as intended

[cwang2016]

All look good except finding two issues,

• In the 'Download Retrieval Script' panel, the option 'Don't unzip the file' for 'Uncompression' is not selectable.
• for the case only one download part shown in 'A sample download', click 'Download Now', the spinning doesn't go away when the download ends.