Do not mangle function names in services using mjml

[ptbrowne]

It is necessary not to mangle classes/function names for mjml to correctly registers its components. Otherwise, mjml2html cannot find its components and fails to render an email (fails silently if the validation mode is not set to true), resulting in a blank email. In banks, the solution was to directly configure cozy-scripts' plugin, but I think we should fix this problem in cozy-scripts. The problem is that I could not get the exclude options to work in cozy-scripts. Thus I don't know how to cleanly disable terser only for mjml here.

       new TerserPlugin({
         parallel: true,
+        exclude: [/mjml/],
         //To fix a SAfari 10 bug : https://github.com/zeit/next.js/issues/5630
         terserOptions: {
           safari10: true
         }
       })

see cozy/cozy-banks#1857