Fix "go back" link in ballot page

Since the `@ballot_referer` variable was only set in the lines
controller, it didn't work when we accessed the ballot page without
adding a line.

Note it still doesn't work if we access the ballot page directly by
entering the URL in the browser's address bar.

app/controllers/budgets/ballot/lines_controller.rb

@@ -7,7 +7,6 @@ module Budgets
       before_action :load_tag_cloud
       before_action :load_categories
       before_action :load_investments
-      before_action :load_ballot_referer
 
       authorize_resource :budget
       authorize_resource :ballot
@@ -67,10 +66,6 @@ module Budgets
           @categories = Tag.category.order(:name)
         end
 
-        def load_ballot_referer
-          @ballot_referer = session[:ballot_referer]
-        end
-
         def load_map
           @investments ||= []
           @investments_map_coordinates = MapLocation.where(investment: @investments).map(&:json_data)

app/views/budgets/ballot/_ballot.html.erb

@@ -1,6 +1,6 @@
 <div class="budget-header">
   <div class="row">
-    <%= back_link_to @ballot_referer %>
+    <%= back_link_to session[:ballot_referer] %>
 
     <h1 class="text-center"><%= t("budgets.ballots.show.title") %></h1>
 

spec/system/budgets/ballots_spec.rb

@@ -422,28 +422,45 @@ describe "Ballots" do
     end
   end
 
-  scenario "Back link after removing an investment from Ballot", :js do
-    create(:budget_investment, :selected, heading: new_york, price: 10, title: "Sully monument")
+  describe "Back link", :js do
+    scenario "after adding and removing an investment from the ballot" do
+      create(:budget_investment, :selected, heading: new_york, price: 10, title: "Sully monument")
 
-    login_as(user)
-    visit budget_investments_path(budget, heading_id: new_york.id)
-    add_to_ballot("Sully monument")
+      login_as(user)
+      visit budget_investments_path(budget, heading_id: new_york.id)
+      add_to_ballot("Sully monument")
 
-    within(".budget-heading") do
-      click_link "Check and confirm my ballot"
+      within(".budget-heading") do
+        click_link "Check and confirm my ballot"
+      end
+
+      expect(page).to have_content("You have voted one investment")
+
+      within(".ballot-list li", text: "Sully monument") do
+        find(".icon-x").click
+      end
+
+      expect(page).to have_content("You have voted 0 investments")
+
+      click_link "Go back"
+
+      expect(page).to have_current_path(budget_investments_path(budget, heading_id: new_york.id))
     end
 
-    expect(page).to have_content("You have voted one investment")
+    scenario "before adding any investments" do
+      login_as(user)
+      visit budget_investments_path(budget, heading_id: new_york.id)
 
-    within(".ballot-list li", text: "Sully monument") do
-      find(".icon-x").click
+      within(".budget-heading") do
+        click_link "Check and confirm my ballot"
+      end
+
+      expect(page).to have_content("You have voted 0 investments")
+
+      click_link "Go back"
+
+      expect(page).to have_current_path(budget_investments_path(budget, heading_id: new_york.id))
     end
-
-    expect(page).to have_content("You have voted 0 investments")
-
-    click_link "Go back"
-
-    expect(page).to have_current_path(budget_investments_path(budget, heading_id: new_york.id))
   end
 
   context "Permissions" do