GUI for configuring payment pointer

[michielbdejong]

In order to make nodeSolidServer/node-solid-server#1513 more usable
(i.e. so that the pod owner does not need to hand-edit json) it would be good if
https://github.com/solid/solid-panes/blob/master/src/dashboard/basicPreferences.ts
had a GUI widget for configuring the pod owner's payment pointer.