move main website from www.s.o to s.o

Our main (wordpress) website is running at www.softwareheritage.org, which is a bit retro for today's standards. We should move it to softwareheritage.org and put redirections in place from www.s.o to s.o.

Addressing this will make #1682 (closed) moot (as long as the //new// redirections work for both http:// and https://, that is).

---

Migrated from T1685 (view on Phabricator)