* Add custom field improvements
This PR does a few things to improve our support of custom fields
1. Adds custom fields added by other plugins to the list in admin/wizards/custom-fields and the custom field list in the mapper selector
2. Adds support for json custom fields in the wizard actions
* Make eslint happy
* Make prettier happy
* Make rubocop happy
* Make ember template lint happy
* Don't assume we have the context in the selector
* Ensure custom fields don't require optional attributes (with tests)
