Add Sluggable concern and unique validation to Budget, Group and Heading

* What: We need to generate slug on Budget, Group and Heading classes, validating its unique for its scope

* How: Adding a presence and unique validation using Budget always as scope.

app/models/budget/group.rb

@@ -1,10 +1,14 @@
 class Budget
   class Group < ActiveRecord::Base
+    include Sluggable
+
     belongs_to :budget
 
     has_many :headings, dependent: :destroy
 
     validates :budget_id, presence: true
-    validates :name, presence: true
+    validates :name, presence: true, uniqueness: { scope: :budget }
+    validates :slug, presence: true, format: /\A[a-z0-9\-_]+\z/
+
   end
-end
+end