Custom field import repair (#3968)

* There is no notes field on accessories.  Fixes Importer Test.

* Fix notification test.  We should see a checkout not allowed exception when trying to check out to a location if the asset requires acceptance.

* Fix Custom field import.

Add a test for custom field import, and fix a few issues related to
importing custom fields.  This will restore v3 functionality.

* Add UI support for mapping custom fields.

This still requires the field mappings to be created/assigned in
advance, but will fetch all custom field names and allow them to be
selected when setting up custom field mappings.

This commit also updates laravel-mix to v1.4.3 and other node
dependencies to fix some build issues.

* Fix some requestable asset page/assetloc issues.  I'd love to know why laravel expections relationships to be in lower case... but thats a question for another day.

app/Http/Transformers/CustomFieldsTransformer.php

@@ -25,7 +25,7 @@ class CustomFieldsTransformer
             'name' => e($field->name),
             'db_column_name' => e($field->db_column_name()),
             'format'   =>  e($field->format),
-            'required'   =>  $field->pivot->required,
+            'required'   =>  $field->pivot ? $field->pivot->required : false,
             'created_at' => Helper::getFormattedDateObject($field->created_at, 'datetime'),
             'updated_at' => Helper::getFormattedDateObject($field->updated_at, 'datetime'),
         ];