Prevent all URL prefixes from being removed from API Backends configuration in the admin

[GUI]

In the API Backends section of the admin, it's required that the backend must have at least 1 URL prefix setup for matching purposes. We added a validation for this condition a while ago, to help ensure admins don't create invalid API backends. This validation is working properly when setting up new API Backends for the first time, but due to some semi-non-intuitive Rails validation behavior, this validation isn't being applied to the update action. This means that it's currently possible to update an existing API backend and remove all of your URL prefixes, which leaves the API backend in an invalid state.

We had an admin hit this condition this week, and to make matters more confusing and worse, the removal of the url prefix meant that the admin no longer had permission to view the API backend (since our permissions are based upon the url prefixes), so they couldn't fix the issue themselves.

We should ensure that on the update action admins can't remove all existing url prefixes which leaves the API in an invalid state. This same issue also affects the list of backend servers that are required too.