Render email content with MJML

[mgax]

Create a Django template tag that renders MJML markup into HTML using the approach in #4.

The NewsletterPageMixin class should provide two new methods:

• get_newsletter_html(): returns the email-friendly HTML representation of the page. It should expect a newsletter_template attribute on the Page class, that contains the name of a Django template, used to render the HTML; if the attribute has not been set, it should throw a useful exception, with guidance on how to fix it.
• get_newsletter_context(): returns the template context for rendering the content. By default it returns {"page": self}. Can be overridden by subclasses.

The result of get_newsletter_html should be displayed in the editor preview, in a new preview mode named Newsletter.

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Here is a minimal page that can be rendered as a newsletter:

class NewsletterPage(NewsletterPageMixin, Page):
    newsletter_template = "myapp/newsletter.html"
    body = RichTextField()

If the user chooses to use django-mjml (or even another rendering system than MJML), they should be able to do so, simply by changing the template.