Move Identity.uniqueness_scope to a module

Moving this method to a separate module looks a bit odd, but it allows
for EE to extend the method without also having to redefine a variety of
validation rules.

app/models/identity.rb

 # frozen_string_literal: true
 
 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: uniqueness_scope, case_sensitive: false }
-  validates :user_id, uniqueness: { scope: uniqueness_scope }
+  validates :extern_uid, allow_blank: true, uniqueness: { scope: UniquenessScopes.scopes, case_sensitive: false }
+  validates :user_id, uniqueness: { scope: UniquenessScopes.scopes }
 
   before_save :ensure_normalized_extern_uid, if: :extern_uid_changed?
   after_destroy :clear_user_synced_attributes, if: :user_synced_attributes_metadata_from_provider?

app/models/identity/uniqueness_scopes.rb

+# frozen_string_literal: true
+
+class Identity < ActiveRecord::Base
+  # This module and method are defined in a separate file to allow EE to
+  # redefine the `scopes` method before it is used in the `Identity` model.
+  module UniquenessScopes
+    def self.scopes
+      [:provider]
+    end
+  end
+end