Closes #14740: Remove BootstrapMixin (#14841)

* Introduce custom form widget templates to apply CSS classes

* Apply both mandatory and optional CSS classes to form widgets

* Omit required & placeholder attrs

* Move annotation of field validation failures to CSS

* Remove BootstrapMixin class

* Remove obsolete ComponentTemplateImportForm class

* Remove obsolete custom forms for login & password change

* Clean up obsolete accommodations for 'required' widget attr

netbox/project-static/styles/transitional/_forms.scss

@@ -16,3 +16,12 @@ form.object-edit {
     content: '\f06C4';
   }
 }
+
+// Set red border on form fields inside a row with .has-errors
+.has-errors {
+  input,
+  select,
+  textarea {
+    border: 1px solid $red;
+  }
+}