mjml: workaround a rendering bug

With the version of mjml we use, trailing text nodes get lost at rendering.
Example: in "<strong>Client</strong>: ACME", the ": ACME" part will be lost.
To avoid this issue, we automatically add a <span/> around such tail text nodes.

The right solution is to update mjml, but it seems to resist me for some
reasons. Until then, we work around.