Make Milestones general, and not specific to Budget Investments

Generalize the Budget::Investment::Milestone model to a
polymorphic Milestone model so it can be used for entities
other than Budget::Investment.

app/models/milestone.rb

@@ -0,0 +1,31 @@
+class Milestone < ActiveRecord::Base
+  include Imageable
+  include Documentable
+  documentable max_documents_allowed: 3,
+               max_file_size: 3.megabytes,
+               accepted_content_types: [ "application/pdf" ]
+
+  translates :title, :description, touch: true
+  include Globalizable
+
+  belongs_to :milestoneable, polymorphic: true
+  belongs_to :status
+
+  validates :milestoneable, presence: true
+  validates :publication_date, presence: true
+  validate :description_or_status_present?
+
+  scope :order_by_publication_date, -> { order(publication_date: :asc, created_at: :asc) }
+  scope :published,                 -> { where("publication_date <= ?", Date.current) }
+  scope :with_status,               -> { where("status_id IS NOT NULL") }
+
+  def self.title_max_length
+    80
+  end
+
+  def description_or_status_present?
+    unless description.present? || status_id.present?
+      errors.add(:description)
+    end
+  end
+end