Add API support for adding and removing Custom Roles

Under https://big.ideas.aha.io/ideas/A-I-9767
partial API support was added to custom user roles.

However this is report only whne the user object is returned. Without the ability to set and unset these custom roles on a user via an API, I'd assert that this capability is very little use in the enterprise where you'll frequently want to tie Aha into your incumbent access management solution.