Issue download modal style #14

What I have done

I redesigned the DownloadModal component.
Now the modal uses the CardHeader, CardContent and CardActions MUI components, just like AddModelGroupModal and EditModelGroupModal.
Also the download button is now variant 'contained', just like the other modals.
I intentionally didnt make DownloadModal the same size as the other modals, since it doesnt have enough content to justify this size.

Screenshot