webgui: flip-flop on old and new URL in case something went wrong
webgui: flip-flop on old and new URL in case something went wrong

The initial timeout of 20 seconds is long, but it's safer to wait so
that we're not bouncing back to the old web GUI before it goes down.

PR: https://github.com/opnsense/core/issues/1347

opnsense/core,opnsense/core,opnsense/core,opnsense/core,opnsense/core,opnsense/core