Image embeding in bulk mailer fails for relative URLs

[jandd]

Describe the bug

If a relative URL is used in mail texts for the admin mail functionality the request handler crashes.

To Reproduce

Steps to reproduce the behavior:

1. Login as admin user
2. Go to /admin/send_email/
3. Select recipient group
4. Fill mail text and include an image with a relative ('/media/...') URL
5. Send test mail
6. See a 404 error

Expected behavior

Relative URLs are handled properly mail is sent.

Additional context

Log contains [ValueError: unknown url type: '/media/...