Update DNS: share becomes share-prev, share-next becomes share

[Paul Chiusano]

Todo:

• create share-prev.unison-lang.org, have that go to same place as current share
• modify share.unison-lang.org to point to same place as current share-next
• add a redirect from share-next.unison-lang.org to share.unison-lang.org in case anyone is hitting share-next urls
• confirm: do push and pull commands work with this config?
• confirm: what about default location where base is fetched from? try creating a fresh empty repo
• confirm: are links on website okay?

[Simon Højberg]

I think this can be closed @pchiusano @aryairani ?