Show email reply to address if one’s been selected

If you’ve picked an email reply to address, it’s good UI design to have
your choice played back to you, so you can be confident it’s worked.
This commit does that by making it part of the email preview.

Uses:
- [ ] https://github.com/alphagov/notifications-utils/pull/227

tests/app/main/views/test_send.py

@@ -16,6 +16,7 @@ from notifications_utils.recipients import RecipientCSV
 
 from tests import validate_route_permission, validate_route_permission_with_client
 from tests.conftest import (
+    fake_uuid,
     mock_get_service_template,
     mock_get_service_template_with_placeholders,
     mock_get_service_letter_template,
@@ -2233,3 +2234,54 @@ def test_send_notification_shows_email_error_in_trial_mode(
     assert normalize_spaces(page.select('.banner-dangerous p')[0].text) == (
         'In trial mode you can only send to yourself and members of your team'
     )
+
+
+@pytest.mark.parametrize('endpoint, extra_args', [
+    ('main.check_messages', {'template_type': 'email', 'upload_id': fake_uuid()}),
+    ('main.send_one_off_step', {'template_id': fake_uuid(), 'step_index': 0}),
+])
+@pytest.mark.parametrize('reply_to_address', [
+    None,
+    fake_uuid(),
+])
+def test_reply_to_is_previewed_if_chosen(
+    client_request,
+    mocker,
+    mock_get_service_email_template,
+    mock_s3_download,
+    mock_get_users_by_service,
+    mock_get_detailed_service_for_today,
+    get_default_reply_to_email_address,
+    endpoint,
+    extra_args,
+    reply_to_address,
+):
+
+    mocker.patch('app.main.views.send.s3download', return_value="""
+        email_address,date,thing
+        notify@digital.cabinet-office.gov.uk,foo,bar
+    """)
+
+    with client_request.session_transaction() as session:
+        session['recipient'] = 'notify@digital.cabinet-office.gov.uk'
+        session['placeholders'] = {}
+        session['upload_data'] = {
+            'original_file_name': 'example.csv',
+            'template_id': fake_uuid(),
+            'notification_count': 1,
+            'valid': True
+        }
+        session['sender_id'] = reply_to_address
+
+    page = client_request.get(
+        endpoint,
+        service_id=SERVICE_ONE_ID,
+        **extra_args
+    )
+
+    email_meta = page.select_one('.email-message-meta').text
+
+    if reply_to_address:
+        assert 'test@example.com' in email_meta
+    else:
+        assert 'test@example.com' not in email_meta