Fix mjml syntax for v4.

[catarak]

Fixes #2303
(Probably, but I haven't checked it with a valid Mailgun config to make sure that it works all the way through to sending the email.)

We merged a dependabot PR #2267 which included a major version bump of mjml and that caused emails to break.

Changes:

• Import mjml2html as the default from mjml instead of a named import.
• Remove deprecated component <mj-container> from templates.
• Convert unitless value width="192" to px width="192px".
• Log errors when calling mjml2html (useful in development, probably does nothing in production but idk).

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123

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