Chosen Widget

We've decided to use a fork of the chosen widget. The code is linked to this PR (https://github.com/harvesthq/chosen/pull/166) which has been maintained over the last 3 years. The feature we needed was the ability to add a new option from user input. This fork does it nicely and didn't require much additional work on our part. It supports adding our own validation before the item is added.

In 4.1, we've started using a modified version as seen in this PR (https://github.com/koenpunt/chosen/pull/48). It adds the ability for the user to press enter to accept the new value which improves the UX.