Closes #12246: General cleanup of utilities modules

* Clean up base modules

* Clean up forms modules

* Clean up templatetags modules

* Replace custom simplify_decimal filter with floatformat

* Misc cleanup

* Merge ReturnURLForm into ConfirmationForm

* Clean up import statements for utilities.forms

* Fix field class references in docs

netbox/utilities/forms/mixins.py

@@ -0,0 +1,62 @@
+from django import forms
+
+from .widgets import APISelect, APISelectMultiple, ClearableFileInput
+
+__all__ = (
+    'BootstrapMixin',
+)
+
+
+class BootstrapMixin:
+    """
+    Add the base Bootstrap CSS classes to form elements.
+    """
+
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+
+        exempt_widgets = [
+            forms.FileInput,
+            forms.RadioSelect,
+            APISelect,
+            APISelectMultiple,
+            ClearableFileInput,
+        ]
+
+        for field_name, field in self.fields.items():
+            css = field.widget.attrs.get('class', '')
+
+            if field.widget.__class__ in exempt_widgets:
+                continue
+
+            elif isinstance(field.widget, forms.CheckboxInput):
+                field.widget.attrs['class'] = f'{css} form-check-input'
+
+            elif isinstance(field.widget, forms.SelectMultiple):
+                if 'size' not in field.widget.attrs:
+                    field.widget.attrs['class'] = f'{css} netbox-static-select'
+
+            elif isinstance(field.widget, forms.Select):
+                field.widget.attrs['class'] = f'{css} netbox-static-select'
+
+            else:
+                field.widget.attrs['class'] = f'{css} form-control'
+
+            if field.required and not isinstance(field.widget, forms.FileInput):
+                field.widget.attrs['required'] = 'required'
+
+            if 'placeholder' not in field.widget.attrs and field.label is not None:
+                field.widget.attrs['placeholder'] = field.label
+
+    def is_valid(self):
+        is_valid = super().is_valid()
+
+        # Apply is-invalid CSS class to fields with errors
+        if not is_valid:
+            for field_name in self.errors:
+                # Ignore e.g. __all__
+                if field := self.fields.get(field_name):
+                    css = field.widget.attrs.get('class', '')
+                    field.widget.attrs['class'] = f'{css} is-invalid'
+
+        return is_valid