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/markdown.py

@@ -1,6 +1,10 @@
 import markdown
 from markdown.inlinepatterns import SimpleTagPattern
 
+__all__ = (
+    'StrikethroughExtension',
+)
+
 STRIKE_RE = r'(~{2})(.+?)(~{2})'