Add missing class to checkbox wrapper

[djw]

From the docs, checkboxes should have the form-check class on their wrapper element, unless the label is omitted:

<div class="form-check">
  <input class="form-check-input" type="checkbox" value="" id="flexCheckDefault">
  <label class="form-check-label" for="flexCheckDefault">
    Default checkbox
  </label>
</div>

<div>
  <input class="form-check-input" type="checkbox" id="checkboxNoLabel" value="" aria-label="...">
</div>

This PR adds this logic.

[smithdc1]

Thanks for the patch, looks good to me.

I'm now thinking about form-horizontal but that's a separate issue!