Navigator FE

[asmega]

Context

• The goal of this PR is to focus on a stateful representation a journey
• Given a set of answers, a pointer to what page the user is on or wants load:
  • are they allowed on the page, not be able to jump to a future page without answering preceding pages ✅
  • determine what the next page should be when they hit continue ✅
  • what the previous page should be when they hit the back link, therefore remove and backlink overrides ✅
  • if a user changes answer retrospectively mid-way thru a journey clear all answers no longer in the "tree" ✅
  • if a user changes answer retrospectively mid-way thru a journey where a later page causes ineligibility clear the last answer in the tree that caused ineligibility ✅
  • this process needs to be deterministic and repeatable ✅
• To limit the amount of work the changes have been constrained to a single journey, FE
• Things to keep in mind but probably out of scope:
  • changing answers on the check answers page affecting state

Notes:

• Every page must now be a form even though that page is simply content
• I've attempted to make the slug sequence additive rather than subtractive. which i think is more logical and easier to reason with. but found this to be more work than is probably worth
• Also resolves https://dfedigital.atlassian.net/browse/CAPT-1912

[rjlynch]

Very nice!

Seems weird that we care about both the current slug and next slug, feel like we only need to care about one of them, the first incomplete slug (ie current slug) - though I appreciate there's a ton of code using current and next slug!

[slorek]

Great work 👏 I like these changes, and it's something that we have been thinking needs to be improved for some time.

I would like to refactor the next_slug and previous_slug methods to be more readable/digestible, and of course we'll need to do some testing of the journey scenarios to make sure nothing is broken.

[github-actions]

Deployments

Journey	URL
Additional Payments	https://claim-additional-payments-for-teaching-review-3522-web.test.teacherservices.cloud/additional-payments/claim
Student Loans	https://claim-additional-payments-for-teaching-review-3522-web.test.teacherservices.cloud/student-loans/claim
Further Education	https://claim-additional-payments-for-teaching-review-3522-web.test.teacherservices.cloud/further-education-payments/landing-page
Early Years Payment	https://claim-additional-payments-for-teaching-review-3522-web.test.teacherservices.cloud/early-years-payment/landing-page
Relocation Payments	https://claim-additional-payments-for-teaching-review-3522-web.test.teacherservices.cloud/get-a-teacher-relocation-payment/landing-page
Admin	https://claim-additional-payments-for-teaching-review-3522-web.test.teacherservices.cloud/admin