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/widgets/misc.py

@@ -0,0 +1,28 @@
+from django import forms
+
+__all__ = (
+    'ClearableFileInput',
+    'MarkdownWidget',
+    'SlugWidget',
+)
+
+
+class ClearableFileInput(forms.ClearableFileInput):
+    """
+    Override Django's stock ClearableFileInput with a custom template.
+    """
+    template_name = 'widgets/clearable_file_input.html'
+
+
+class MarkdownWidget(forms.Textarea):
+    """
+    Provide a live preview for Markdown-formatted content.
+    """
+    template_name = 'widgets/markdown_input.html'
+
+
+class SlugWidget(forms.TextInput):
+    """
+    Subclass TextInput and add a slug regeneration button next to the form field.
+    """
+    template_name = 'widgets/sluginput.html'