Add `returnUrl` to form page config

[Mottie]

Issue Description

In a CustomPage, the setFormData function also triggers an auto save which sets the returnUrl to the current path. If that CustomPage is usually hidden (depends callback always returns false), then if the user closes the browser and later returns, the returnUrl will point to a hidden page and the save in progress continuation will cause the button click to return to the introduction page. There is no way to fix this "broken" returnUrl without restarting the form.

Front end tasks

• [ ] Add a method to set the returnUrl for hidden Custom pages
• [ ] If the "Finish this application later" link is visible & used, it also uses the set returnUrl
• [ ] Check the review & submit page auto save
• [ ] Add unit tests
• [ ] Request to add documentation

Acceptance Criteria

• [ ] All tests passing
• [ ] Include documentation

[saderagsdale]

@Mottie this wasn't added to the sprint plan. I'll add it.