Add slug string column to Budget, Budget::Heading and Budget::Group

Why:

* We're still using id's on routes and other parts of the app, using slugs are more friendly and nice

How:

* Just a migration with a slug column with string type

db/migrate/20170704105112_add_slugs_to_budget_heading_group.rb

@@ -0,0 +1,7 @@
+class AddSlugsToBudgetHeadingGroup < ActiveRecord::Migration
+  def change
+    add_column :budgets, :slug, :string
+    add_column :budget_groups, :slug, :string
+    add_column :budget_headings, :slug, :string
+  end
+end