Consolidate functionality shared between Issue and MergeRequest

Any associations, validations, delegates, scopes and methods that
were exactly the same in both Issue and MergeRequest models have been
moved to a new IssueCommonality module (role) that gets included by each
class.

There was actually quite a bit of duplication, because MergeRequests are
basically just specialized Issues.

app/models/issue.rb

@@ -4,58 +4,17 @@ class Issue < ActiveRecord::Base
 
   acts_as_taggable_on :labels
 
-  belongs_to :project
   belongs_to :milestone
-  belongs_to :author, :class_name => "User"
-  belongs_to :assignee, :class_name => "User"
-  has_many :notes, :as => :noteable, :dependent => :destroy
-
-  attr_protected :author, :author_id, :project, :project_id
-  attr_accessor :author_id_of_changes
-
-  validates_presence_of :project_id
-  validates_presence_of :author_id
-
-  delegate :name,
-           :email,
-           :to => :author,
-           :prefix => true
-
-  delegate :name,
-           :email,
-           :to => :assignee,
-           :allow_nil => true,
-           :prefix => true
-
-  validates :title,
-            :presence => true,
-            :length   => { :within => 0..255 }
 
   validates :description,
             :length   => { :within => 0..2000 }
 
-  scope :opened, where(:closed => false)
-  scope :closed, where(:closed => true)
-  scope :assigned, lambda { |u| where(:assignee_id => u.id)}
-
   acts_as_list
 
   def self.open_for(user)
     opened.assigned(user)
   end
 
-  def self.search query
-    where("title like :query", :query => "%#{query}%")
-  end
-
-  def today?
-    Date.today == created_at.to_date
-  end
-
-  def new?
-    today? && created_at == updated_at
-  end
-
   def is_assigned?
     !!assignee_id
   end