Fix crash voting on a heading with a content block

When voting investment projects, the sidebar was rendered without the
`@heading_content_blocks` being set. That resulted in a 500 error when
the heading had content blocks.

By extracting the logic to a component, we make sure the heading content
blocks are properly set every time this code is rendered, no matter
which controller is rendering the view.

spec/system/budgets/ballots_spec.rb

@@ -160,13 +160,16 @@ describe "Ballots" do
         end
       end
 
-      scenario "the Map shoud be visible before and after" do
+      scenario "map and content block shoud be visible before and after" do
         create(:budget_investment, :selected, heading: new_york, price: 10000, title: "More bridges")
+        create(:heading_content_block, heading: new_york, body: "<li>New Block</li>")
+        new_york.update!(allow_custom_content: true)
 
         visit budget_investments_path(budget, heading_id: new_york)
 
         within("#sidebar") do
           expect(page).to have_content "OpenStreetMap"
+          expect(page).to have_content "New Block"
         end
 
         add_to_ballot("More bridges")
@@ -174,6 +177,7 @@ describe "Ballots" do
         within("#sidebar") do
           expect(page).to have_content "More bridges"
           expect(page).to have_content "OpenStreetMap"
+          expect(page).to have_content "New Block"
         end
 
         within(".budget-investment", text: "More bridges") do
@@ -183,6 +187,7 @@ describe "Ballots" do
         within("#sidebar") do
           expect(page).not_to have_content "More bridges"
           expect(page).to have_content "OpenStreetMap"
+          expect(page).to have_content "New Block"
         end
       end
     end