Change `group` association to `namespace`

This is more accurate, as it can reference a
top-level namespace.

app/models/work_item/type.rb

@@ -13,7 +13,7 @@ class WorkItem::Type < ApplicationRecord
 
   enum base_type: Issue.issue_types
 
-  belongs_to :group, foreign_key: :namespace_id, optional: true
+  belongs_to :namespace, optional: true
   has_many :work_items, class_name: 'Issue', foreign_key: :work_item_type_id, inverse_of: :work_item_type
 
   before_validation :strip_whitespace

spec/factories/work_item/work_item_types.rb

@@ -2,10 +2,14 @@
 
 FactoryBot.define do
   factory :work_item_type, class: 'WorkItem::Type' do
-    group
+    namespace
 
     name      { generate(:work_item_type_name) }
     icon_name { 'issue' }
     base_type { Issue.issue_types['issue'] }
+
+    trait :default do
+      namespace { nil }
+    end
   end
 end

spec/models/work_item/type_spec.rb

@@ -9,7 +9,7 @@ RSpec.describe WorkItem::Type do
 
   describe 'associations' do
     it { is_expected.to have_many(:work_items).with_foreign_key('work_item_type_id') }
-    it { is_expected.to belong_to(:group).with_foreign_key('namespace_id') }
+    it { is_expected.to belong_to(:namespace) }
   end
 
   describe '#destroy' do