Spike: investigate ways to render mail content

[mgax]

I've played around with the template rendering for the newsletter and have this experiment working: https://github.com/mgax/newsletter-spike-mailchimp/blob/55a0996acca30938a77aef7c489c66d6b53f6a98/spike/content/models.py#L95-L102.

• The integrator needs to subclass their page from NewsletterPageMixin and define a newsletter_template.
• The template should include a footer. It's up to the integrator to inline the footer or pull it from elsewhere.
• The mixin takes care of invoking mrml.to_html to render the template. If the integrator wants to replace the MJML implementation, they can override render_newsletter, which is quite small.

[mgax]

TODO:

• See how somebody might swap in https://pypi.org/project/django-mjml/.
• See how somebody might write the same MJML processing, regardless of whether they are writing a template for NewsletterPageMixin as opposed to a Django email template outside of Wagtail.

[mgax]

@thibaudcolas I've changed the spike implementation to do the MJML rendering inside the template with an {% mrml %} tag that is a drop-in replacement for django-mjml's {% mjml %} tag: https://github.com/mgax/newsletter-spike-mailchimp/commit/69e3a0c0ebc791f39de2a59219832a214c58b339.