Extract BudgetHeadingsHelper#heading_link & use it

Why:

The logic to construct the link to a heading (if it exists) is in three
different places, this is a clear candidate for a helper method.

How:

Just checking at the helper method if `assigned_heading` and `budget`
has values and composing the link if so.

app/helpers/budget_headings_helper.rb

@@ -6,4 +6,10 @@ module BudgetHeadingsHelper
     end
   end
 
+  def heading_link(assigned_heading = nil, budget = nil)
+    return nil unless assigned_heading && budget
+    heading_path = budget_investments_path(budget, heading_id: assigned_heading.try(:id))
+    link_to(assigned_heading.name, heading_path)
+  end
+
 end