Update editor state by returning Promise from onSave function

Created by: pjlast

Changes the onSave function to return a Promise. This Promise can be awaited by the original caller and allows us to react on the site config as returned back from the server.

Fix for https://github.com/sourcegraph/customer/issues/979

Test plan

Manual e2e test since this is a UX issue

https://www.loom.com/share/5be1bd53db7040d4a58ffdf1b85346a2

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.