Merge branch 'ce-jej/identity-relates-to-saml' into 'master'

[CE] Backport Identity.uniqueness_scope from EE

See merge request gitlab-org/gitlab-ce!18689

app/models/identity.rb

 class Identity < ActiveRecord::Base
+  def self.uniqueness_scope
+    :provider
+  end
+
   include Sortable
   include CaseSensitivity
   belongs_to :user
   validates :provider, presence: true
-  validates :extern_uid, allow_blank: true, uniqueness: { scope: :provider, case_sensitive: false }
-  validates :user_id, uniqueness: { scope: :provider }
+  validates :extern_uid, allow_blank: true, uniqueness: { scope: uniqueness_scope, case_sensitive: false }
+  validates :user_id, uniqueness: { scope: uniqueness_scope }
   before_save :ensure_normalized_extern_uid, if: :extern_uid_changed?
   after_destroy :clear_user_synced_attributes, if: :user_synced_attributes_metadata_from_provider?