Support HTTPS

[tuxayo]

https://live.openstreetmap.fr/ doesn't work.
Due to these two lines https://github.com/cstenac/osm-livechanges/blob/master/frontend/index.html#L8

https://cdn.leafletjs.com doesn't work with HTTPS.

One solution would be not use a CDN [1] and to host the libs directly. (using a dependency manager like bower, npm, yarn to not throw libs into the git repo)

[1] which has also other advantages
https://www.peppercarrot.com/en/article390/my-fight-against-cdn-libraries
http://braincracking.org/2016/12/21/combien-coute-un-service-tiers-gratuit

[Lee-Carre]

https://live.openstreetmap.fr/ doesn't work.

Seems that this has since been resolved.