misc: add django widget type as css class (#46440)

gadjo/templates/gadjo/widget.html

@@ -1,6 +1,7 @@
-{% load i18n %}
+{% load gadjo i18n %}
 <div class="widget
             {{ field.css_classes }}
+            django-{{ field|field_class_name }}
             {% if field.errors %}widget-with-error{% endif %}
             {% if field.field.required %}widget-required{% else %}widget-optional{% endif %}"
      id="{{field.id_for_label}}_p">

gadjo/templatetags/gadjo.py

@@ -7,6 +7,7 @@ from xstatic.main import XStatic
 from django import template
 from django.conf import settings
 from django.core.exceptions import ImproperlyConfigured
+from django.forms import BoundField
 from django.utils.html import escape
 from django.utils.http import urlencode
 
@@ -122,3 +123,15 @@ def querystring(parser, token):
 def with_template(form):
     form_template = template.loader.get_template('gadjo/form.html')
     return form_template.render({'form': form})
+
+
+# pattern to transform Django camel case class names to CSS class names with
+# dashes. (CheckboxInput -> checkbox-input)
+class_name_pattern = re.compile(r'(?<!^)(?=[A-Z])')
+
+
+@register.filter
+def field_class_name(field):
+    if isinstance(field, BoundField):
+        field = field.field
+    return class_name_pattern.sub('-', field.widget.__class__.__name__).lower()