Cloud - Attempt to Create User With Existing Email Hangs Indefinitely

When attempting to create a new User in cloud and the Email entered is already being used for another user, there is no in form validation and "Create user" can be clicked. When this is done an error is displayed, but when dismissed the "Create User" and "Cancel" buttons no longer work and a loading icon is displayed indefinitely.

Appears cloud specific where we've removed the "Username" field and are using the "Email" field as both email/username. In on-prem the username field is validated in real time and does not allow you to click "Create user" if the username already exists.

https://github.com/Graylog2/graylog2-server/assets/103449971/817ebc02-ba49-4c10-9fa6-75b46550a657

## Expected Behavior

Validation of Email field for uniqueness before allowing form submit.

## Current Behavior

## Possible Solution

Validation of Email field for uniqueness before allowing form submit.

## Steps to Reproduce (for bugs)

See video.

## Context

## Your Environment

* Graylog Version:
* Java Version:
* OpenSearch Version:
* MongoDB Version:
* Operating System:
* Browser version: