Upload license file through Panel

You can download a license file from our license center.
Would be neat if you could just drop this into the Panel to upload it in the right place.

Context: https://github.com/getkirby/kirby/issues/1831#issuecomment-542214520

I think we can consider this as completed given that one can either activate the site right through the Panel and/or download and copy the license file manually in the right place.