Move EE code from Issue to EE::Issue

This moves EE specific code from the Issue model into the EE::Issue
module, reducing the chance of developers running into merge conflicts.

app/models/issue.rb

@@ -3,16 +3,12 @@
 require 'carrierwave/orm/activerecord'
 
 class Issue < ActiveRecord::Base
-  prepend EE::Issue
-  prepend EE::RelativePositioning
-
   include AtomicInternalId
   include IidRoutes
   include Issuable
   include Noteable
   include Referable
   include Spammable
-  include Elastic::IssuesSearch
   include FasterCacheKeys
   include RelativePositioning
   include TimeTrackable
@@ -45,9 +41,6 @@ class Issue < ActiveRecord::Base
   has_many :issue_assignees
   has_many :assignees, class_name: "User", through: :issue_assignees
 
-  has_one :epic_issue
-  has_one :epic, through: :epic_issue
-
   validates :project, presence: true
 
   alias_attribute :parent_ids, :project_id
@@ -313,3 +306,5 @@ class Issue < ActiveRecord::Base
     Gitlab::EtagCaching::Store.new.touch(key)
   end
 end
+
+Issue.prepend(EE::Issue)

ee/app/models/ee/issue.rb

@@ -9,9 +9,15 @@ module EE
       WEIGHT_ANY = 'Any'.freeze
       WEIGHT_NONE = 'None'.freeze
 
+      prepend EE::RelativePositioning
+      include Elastic::IssuesSearch
+
       scope :order_weight_desc, -> { reorder ::Gitlab::Database.nulls_last_order('weight', 'DESC') }
       scope :order_weight_asc, -> { reorder ::Gitlab::Database.nulls_last_order('weight') }
 
+      has_one :epic_issue
+      has_one :epic, through: :epic_issue
+
       validates :weight, allow_nil: true, numericality: { greater_than_or_equal_to: 0 }
     end