checkboxes are not rendered in data.html.twig email body when used in fieldsets

Like I mentioned in another issue, something seems broken with the `checkbox.html.twig` template.
I define checkbox fields like this in my `form.md`

```
beratung:
type: checkbox
outerclasses: px1 w12 mb1
label: "Beratung vor Ort"
```
But when sending the email, they will not appear in the email body.

Start by reading the checkbox.html.twig and data.html.twig templates, then reproduce the issue with a checkbox field inside a fieldset in form.md. Compare the generated email body with the submitted form data. Done means checkbox fields used in fieldsets appear correctly in the email body.

Written by the indexing model from the issue text.