Replace nodejs based MJML with rust variant, MRML

[mrchrisadams]

We currently rely on a version of MJML that relies on calling a nodejs binary installed on deployment. you can see it specificed in the base.py:

MJML_CHECK_CMD_ON_STARTUP = True
MJML_PATH = str(PROJECT_DIR / "theme" / "static_src" / "node_modules/.bin/mjml")
MJML_EXEC_CMD = [MJML_PATH, "--config.validationLevel", "skip"]

This can be a bit awkward to work with, and there's a new library, mjml-python, that wraps MRML the rust implementation of the MJML compiler. This means we'd no longer have the dependency on an external node process, and likely would mean writing email ready HTML files from MJML would be faster and more resources efficient

There's an existing issue discussing this in more detail with sample code to use as well?

liminspace/django-mjml#184

The efficiency / performance uplift is pretty impressive - this comment when someone else tried the approach described in the issue was quite eye opening:

Tried @blopker version. Works well so far, and about 100x faster than cmd implementation of django-mjml. 0.006 seconds per render vs 0.9 seconds

Source: Integrate mjml-python? · Issue #184 · liminspace/django-mjml by @github