Refactor form step validation as required field

[aduth]

Why: Reduces responsibility of step where "is required" is the only validation currently in use, and allows validation of conditionally-registered fields.

Specifically, this would be needed for the use-case described at #4237 (comment), where in the review screen introduced in #4237, the "Selfie" field is required, but only if liveness checking is enabled for the service provider. The current step validation behavior has no direct awareness of whether liveness is enabled.

Alternatives considered:

• It could be possible to pre-bind a validation function with an argument controlling whether selfie should be checked.

const reviewStep = { validate: validateReviewStep.bind(null, isLivenessEnabled);
// ...
function validate(isLivenessEnabled, values) {
  const valuesToCheck = isLivenessEnabled ? ['front', 'back', 'selfie'] : ['front', 'back'];
  // ...
}

Note that some of the changes here assume or otherwise depend on changes from #4237 (e.g. initialStep -> autoFocus).

[aduth]

Note that this also removes logic in FormSteps which would feasibly have allowed a user to pick up from where they left off after a reload. We don't currently take advantage of this, and it's not clear that we would care to. There is a low-priority ticket LG-3335 which would aim to allow this, though it may be very incompatible with other work with async uploads and had been contemplated as being cancelled altogether.

[zachmargolis]

LGTM

[aduth]

Possible regression from these changes: I've noticed in the mobile flow that immediately after selecting a "Front of your ID" image, the "Back of your ID" field is flagged as "This field is required". The error should only be shown if the user attempts to continue to the next step before providing a value.

[aduth]

The issue described at #4243 (comment) is related to this, where in toNextStep we will always assign setActiveErrors(getValidationErrors()). In the mobile flow this happens when continuing from the first "Mobile Intro" step. There are no actual errors, but it still sets an empty array, which is considered truthy by if (activeErrors) {. The idea with this code was to try to "chip away at" any active errors.

Possible fixes:

• Change if (activeErrors) { to if (activeErrors?.length)
• Guard setActiveErrors in toNextStep to only set active errors if non-empty
• Return undefined from getValidationErrors if result would be empty