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?