Locked email content includes preview header on live send

[cjg89]

Describe the bug

See title.

Steps to Reproduce

1. Create an email with preview enabled.
2. Wait for the preview to send.
3. Lock the email content before the live send.
4. Verify the live send includes the black preview bar.

Expected Behavior

Live sends should never include preview black bars.

Possible Fix

In Email.send_preview(), make sure the sent_html in the created PreviewInstance object does not include preview headers.

Your environment:

n/a

Additional context

n/a