Duplicating an email forgets the theme

[richardolsson]

Description

When using the "Duplicate" feature under the ellipsis menu for an email, the new (cloned) email is not configured to use the same theme, so it appears different.

Steps to reproduce

1. Go to https://app.dev.zetkin.org/organize/1/projects
2. In any campaign, create an email (which will use the default theme)
3. In the ellipsis menu, choose "Duplicate"
4. Look at the data for the new email (e.g. using /api/orgs/{ORG_ID}/emails/{EMAIL_ID}

Expected Behaviour

Data should contain reference to same theme

Actual Behaviour

Data does not contain a theme.

Screenshots (if you have any)

N/A

Proposed solution

I'm pretty sure it's just a matter of include the theme_id when cloning, the same way that content, title etc are being included.