`form-control` css class added to checkbox when `form_show_labels == False`

[powderflask]

A Checkbox input should not have the form-control css class as bootstrap3 adds height=34px to form-control elements.

crispy-forms 1.14 introduced a bug in bootstrap3/field.html template that causes the form-control css class to be added to the input element when form_show_labels == False

To reproduce, create a form with:

my_checkbox = forms.BooleanField(required=False, widget=forms.CheckboxInput())
form.helper.form_show_labels = False

Rendered field has form-control css class, which styles a form-control-sized checkbox 34px tall.

Use case: a formset view where items can be selected (i.e., each checkbox has no label)

This is easy to fix. I'll follow up with a PR shortly.