Move setting slug to after action

Adjusts the completed slugs method to only update the list of visited
slugs if the form is valid.

There's been some rollbar errors where the user seems to have skipped
ahead in the journey but hasn't be redirected back by the
check_page_is_in_sequence callback.

All the tests pass but this seems like it could be risky. I'm not 100%
sure that no code is subtly relying on the visited slugs being updated
on failed form verifications.

We may want to consider a different approach to wizard state management,
maybe passing the session answers to a state machine to determine which
slugs are accessible.

app/controllers/claims_controller.rb

@@ -4,13 +4,13 @@ class ClaimsController < BasePublicController
   skip_before_action :send_unstarted_claimants_to_the_start, only: [:new, :create]
   before_action :initialize_session_slug_history
   before_action :check_page_is_in_sequence, only: [:show, :update]
-  before_action :update_session_with_current_slug, only: [:update]
   before_action :set_backlink_path, only: [:show, :update]
   before_action :check_claim_not_in_progress, only: [:new]
   before_action :clear_claim_session, only: [:new], unless: -> { journey.start_with_magic_link? }
   before_action :prepend_view_path_for_journey
   before_action :persist_claim, only: [:new, :create]
   before_action :handle_magic_link, only: [:new], if: -> { journey.start_with_magic_link? }
+  after_action :update_session_with_current_slug, only: [:update]
 
   include AuthorisedSlugs
   include FormSubmittable
@@ -90,7 +90,11 @@ def initialize_session_slug_history
   end
 
   def update_session_with_current_slug
-    session[:slugs] << params[:slug] unless Journeys::PageSequence::DEAD_END_SLUGS.include?(params[:slug])
+    if @form.nil? || @form.valid?
+      session[:slugs] << params[:slug] unless Journeys::PageSequence::DEAD_END_SLUGS.include?(params[:slug])
+    else
+      # Don't count form as visited if it's invalid
+    end
   end
 
   def check_claim_not_in_progress