Fix export kml in safari browsers

This is a fix for https://github.com/geoadmin/mf-geoadmin3/issues/1785

As written in [1] , this is quite a pain. I was able to fix this by using an older blob.js file.

As safari does not support naming a file [2] , the generated kml is opened in a new tab. The user then has to manually use the 'save as...' function of the browser to save it to disk. Another variant would be to download it directly, but with a generic name like `Unknow` without any extension.

Behaviour on other browsers should not be affected.

[1] https://github.com/eligrey/FileSaver.js/issues/12
[2] https://github.com/eligrey/FileSaver.js