Confirm recipient’s email address by default

This matches how the API behaves – email confirmation is on unless the
calling service sets the argument to `False`:
https://github.com/alphagov/notifications-api/blob/0315a5be585472700c509f65224898d183e2e048/app/v2/notifications/post_notifications.py#L271

It also follows what we say in the docs, that turning off this feature
is ‘not recommended’:
https://github.com/alphagov/notifications-tech-docs/blob/712ed70fe68ebe77741c5983558c58bfeb25a0c2/source/rest-api.html.md.erb#L472

Author: quis

app/notify_client/template_email_file_client.py

@@ -18,7 +18,7 @@ def create_file(
         filename,
         created_by_id,
         retention_period=90,
-        validate_users_email=None,
+        validate_users_email=True,
     ):
         data = {
             "id": str(file_id),

tests/app/main/views/test_template_email_files.py

@@ -358,7 +358,7 @@ def test_upload_file_does_not_update_template_when_placeholder_already_exists(
                 "filename": "tests/test_pdf_files/one_page_pdf.pdf",
                 "created_by_id": AnyStringMatching(UUID4_REGEX_PATTERN),
                 "retention_period": 90,
-                "validate_users_email": False,
+                "validate_users_email": True,
             },
         ),
     ]

tests/app/notify_client/test_template_email_files_client.py

@@ -32,5 +32,5 @@ def test_create_file_calls_endpoint_with_correct_data(mocker, data):
     client.create_file(**data, service_id=service_id, template_id=template_id)
     data["id"] = data.pop("file_id")
     if "validate_users_email" not in data.keys():
-        data["validate_users_email"] = False
+        data["validate_users_email"] = True
     mock_post.assert_called_once_with(expected_url, data=data)