Hide group name only on budgets with one group

In the form of creating a new investment was hiding the name of the
group if it had only one heading, but could be confusing to users if
there are, for example, five different groups of one heading.

The solution:

- If the budget has one group and one heading, the heading selector is
  hidden.
- If the budget has one group and more than one heading, the group name
  is hidden.
- If the budget has more than one group, the group name appears
  regardless of the number of headings.

spec/system/budgets/investments_spec.rb

@@ -570,18 +570,35 @@ describe "Budget Investments" do
       expect(page).to have_content "Build a skyscraper"
     end
 
-    scenario "Create with multiple headings" do
-      create(:budget_heading, budget: budget, name: "Medical supplies")
-      create(:budget_heading, budget: budget, name: "Even more hospitals")
+    scenario "Create with single group and multiple headings" do
+      create(:budget_heading, group: group, name: "Medical supplies")
+      create(:budget_heading, group: group, name: "Even more hospitals")
+
+      login_as(author)
+
+      visit new_budget_investment_path(budget)
+
+      expect(page).to have_select "Heading",
+        options: ["", "More hospitals", "Medical supplies", "Even more hospitals"]
+      expect(page).not_to have_content "Health"
+    end
+
+    scenario "Create with multiple groups" do
+      education = create(:budget_group, budget: budget, name: "Education")
+
+      create(:budget_heading, group: group, name: "Medical supplies")
+      create(:budget_heading, group: education, name: "Schools")
+
       login_as(author)
 
       visit new_budget_investment_path(budget)
 
       expect(page).not_to have_content("#{heading.name} (#{budget.formatted_heading_price(heading)})")
       expect(page).to have_select "Heading",
-        options: ["", "More hospitals", "Medical supplies", "Even more hospitals"]
+        options: ["", "Health: More hospitals", "Health: Medical supplies", "Education: Schools"]
+
+      select "Health: Medical supplies", from: "Heading"
 
-      select "Medical supplies", from: "Heading"
       fill_in "Title", with: "Build a skyscraper"
       fill_in_ckeditor "Description", with: "I want to live in a high tower over the clouds"
       fill_in "Location additional info", with: "City center"
@@ -755,7 +772,7 @@ describe "Budget Investments" do
 
       select_options = find("#budget_investment_heading_id").all("option").map(&:text)
       expect(select_options).to eq ["",
-                                    "Toda la ciudad",
+                                    "Toda la ciudad: Toda la ciudad",
                                     "Health: More health professionals",
                                     "Health: More hospitals"]
     end