older dartpad domains should redirect to dartpad.dev

[jamesderlin]

Currently https://dartpad.dartlang.org responds with a 404 page. https://dartpad.dartlang.org was the URL for quite a while and now is ingrained into my browser history, so whenever I type "dartpad" into my address bar, it autocompletes to the old address.

It would be nice if it responded with a 308 (or perhaps 301) permanent redirect to https://dartpad.dev/.

[parlough]

Even though DartPad hasn't used the URL in a long time, there are still links around, so a permanent redirect sounds good.

[parlough]

\cc @johnpryan Would you be able to add this redirect? Do you have access?

[devoncarew]

See also #2646 for the www.dartpad.dev domain.