IdV API: Set last GPO code for auto-fill in local development

[aduth]

Why: So that a developer can complete the GPO proofing flow locally.

Testing Instructions:

1. Go to http://localhost:3000
2. Sign in
3. Go to http://localhost:3000/verify
4. Complete proofing flow, opting to verify address by mail at the phone step
5. On "Come back later" screen, click Continue
6. On account screen, click to enter the GPO code received by mail
7. Observe there is a code pre-filled
8. Submit the form
9. Observe the profile is confirmed

Notes:

Referenced here:

identity-idp/app/controllers/idv/gpo_verify_controller.rb

Line 13 in 1e0f764

@code = session[:last_gpo_confirmation_code] if FeatureManagement.reveal_gpo_code?

Assigned in previous flow here:

identity-idp/app/controllers/idv/review_controller.rb

Lines 52 to 53 in 1e0f764

	return unless FeatureManagement.reveal_gpo_code?
	session[:last_gpo_confirmation_code] = idv_session.gpo_otp

[zachmargolis]

LGTM