Add edit_folder_permissions permission

Admin part of alphagov/notifications-api#2356
Adds Folder permissions switch to the platform admin part of service
settings page.

tests/app/main/views/service_settings/test_service_setting_permissions.py

@@ -46,6 +46,8 @@ def test_service_set_permission_requires_platform_admin(
     ('inbound_sms', 'False', False),
     ('email_auth', 'True', True),
     ('email_auth', 'False', False),
+    ('edit_folder_permissions', 'True', True),
+    ('edit_folder_permissions', 'False', False),
 ])
 def test_service_set_permission(
     mocker,

tests/app/main/views/test_service_settings.py

@@ -102,6 +102,7 @@ def mock_get_service_settings_page_common(
         'Receive inbound SMS Off Change',
         'User auth type editing Off Change',
         'Uploading documents Off Change',
+        'Folder permissions Off Change',
 
     ]),
 ])