Hide max votable field on single heading budgets

IMHO selecting in how many headings it's possible to support investments
isn't necessary when there's only one option to choose from. It's
obvious that if there's only one heading, it will be impossible to
select investments from more than one heading.

app/models/budget/group.rb

@@ -30,6 +30,10 @@ class Budget
       all.sort_by(&:name)
     end
 
+    def multiple_headings?
+      headings.size > 1
+    end
+
     private
 
       def generate_slug?

app/views/admin/budget_groups/_form.html.erb

@@ -11,7 +11,7 @@
     </div>
   <% end %>
 
-  <% if group.persisted? && group.headings.any? %>
+  <% if group.multiple_headings? %>
     <div class="small-12 medium-6 column margin">
       <%= f.select :max_votable_headings,
                    (1..group.headings.count),

spec/system/admin/budget_groups_spec.rb

@@ -129,14 +129,23 @@ describe "Admin budget groups", :admin do
       expect(page).to have_field "Maximum number of headings in which a user can select projects", with: "2"
     end
 
-    describe "Select for maxium number of headings to select projects" do
-      scenario "is present if there are headings in the group" do
+    describe "Select for maximum number of headings to select projects" do
+      scenario "is present if there are several headings in the group" do
+        group = create(:budget_group, budget: budget)
+        2.times { create(:budget_heading, group: group) }
+
+        visit edit_admin_budget_group_path(budget, group)
+
+        expect(page).to have_field "Maximum number of headings in which a user can select projects"
+      end
+
+      scenario "is not present if there's only one heading in the group" do
         group = create(:budget_group, budget: budget)
         create(:budget_heading, group: group)
 
         visit edit_admin_budget_group_path(budget, group)
 
-        expect(page).to have_field "Maximum number of headings in which a user can select projects"
+        expect(page).not_to have_field "Maximum number of headings in which a user can select projects"
       end
 
       scenario "is not present if there are no headings in the group" do