Create Live Media should handle split ISOs

We are now splitting ISOs when they become larger than 2 GB. The __Create Live Media__ utility should be able to download and write such ISOs.

Manual workaround:

```
% sudo umount /dev/da0*
% cat hello-0.5.0_0E63-FreeBSD-13.0-amd64.iso.part1 >> hello-0.5.0_0E63-FreeBSD-13.0-amd64.iso.part0
% sudo dd if=hello-0.5.0_0E63-FreeBSD-13.0-amd64.iso.part0 of=/dev/da0 bs=4m status=progress
```

Possible approach:

```
import urllib.request
import shutil

self.current_offset = 0 # Increment this after each part that is being written

filename = archive[i]
with urllib.request.urlopen(url) as response, open(filename, 'wb') as out_file:
fileObject.seek(self.current_offset, 0)
shutil.copyfileobj(response, out_file)
```

As a side effect of using `shutil.copyfileobj` we should also support writing **local ISO files** and **local split ISO files** like we currently do for `Other...` ISO URLs.