Plans page: more upgrade options

The plans page has two changes. Members on an older Hobbyist or Founder plan can now move up to Builder or a bigger plan from Account > Billing > Plans, with credit for the time already paid. The page also lists the biggest plan first. Each upgrade card now shows roughly what the upgrade will cost you today, so you can see the charge before you click. While the estimate loads, the card still says you pay only the difference today.