Allow mailto link in email (button or link)

[richardolsson]

Description

One user wants to be able to add a link to an email address in a mass email. They want to write some text, and then include a link in that text (or in a standalone button block) to some email address using a mailto: link, but that does not currently work.

Steps to reproduce

1. Go to https://app.dev.zetkin.org/organize/1
2. Go into any project or create a new one
3. Create an email
4. Go to the "Compose" tab
5. In the composer, add a button
6. Edit the URL of the button to be mailto:info@example.com
7. Click "Click to test link"

Expected Behaviour

The link should open an email client (or however the browser is configured to handle mailto links).

Actual Behaviour

The link takes you to http://DOMAIN where DOMAIN is the domain part of the email address.

Screenshots (if you have any)