Base getLoadValue to look at form field first

Fixes https://github.com/octobercms/october/pull/2663
Adding to build 420+ because this might cause some issues, although it shouldn't. Hoping we don't ever have to roll this back because it fixes another inconsistency when using the model `filterFields` method... some will change values via $field->value (field accessor) and others will change via $this->value ($model accessor). This now puts the field accessor at a consistent priority (first) while retaining the fallback to model.

modules/backend/classes/FormWidgetBase.php

@@ -123,6 +123,10 @@ abstract class FormWidgetBase extends WidgetBase
      */
     public function getLoadValue()
     {
+        if ($this->formField->value !== null) {
+            return $this->formField->value;
+        }
+
         $defaultValue = !$this->model->exists
             ? $this->formField->getDefaultFromData($this->data ?: $this->model)
             : null;