Fix investments link in single heading budgets

The link to "See all investments" didn't have the `heading_id`
parameter, which resulted in the ballot information not being displayed
when in the voting phase.

We could modify the link to include the `heading_id` parameter, but IMHO
it's more robust to select the heading automatically when there's only
one heading. That way manually accessing the page without a `heading_id`
parameter will still work as if the heading had been selected.

spec/system/budgets/investments_spec.rb

@@ -1356,6 +1356,9 @@ describe "Budget Investments" do
         expect(page).to have_content investment2.title
         expect(page).to have_content "€20,000"
       end
+
+      expect(page).to have_link "Submit my ballot"
+      expect(page).to have_content "STILL AVAILABLE TO YOU €666,666"
     end
 
     scenario "Order by cost (only when balloting)" do