pick canonical website domain and enforce it

We need to pick either www.mesa3d.org or mesa3d.org and redirect the other one (most likely via HTTP 301 on the nginx reverse proxy).

As far as I can tell, the arguments are:

• www. has been so (over)used that some people are used to it and expect it (haven't seen any data backing this though)
• the fact it's a website is no longer conveyed by having a server called "world wide web" and this relic of the past has no meaning anymore (https:// serves that purpose, not to mention people know what a domain is by now, especially among the crowd that goes on a graphics driver's website)
• www. makes the URL longer (yeah, I know most people don't care about that)

I'm (probably obviously) in favour of picking mesa3d.org as the canonical website domain, but I want everyone who cares about this to chime in :)

/cc @daniels for the technical side of things