provide french translation

I'm having trouble using Mailman with french-speaking users, naturally, as templates and GUI translations seem to be completely missing.

Mailman 2 was, as far as I remember, fairly well translated to french - it would be a shame to just drop that work in the upgrade, as many english templates have just been copied over, from what I can tell.

This is of course a problem with many other languages (see for example german in #534 (closed)) but I figured I would start with my own itch and see where it goes. :)