text: Title: UI Bug when trying to add user with email taken on Admin Panel
  Body: **Environment**:
  
  - CTFd Version/Commit: 3.2.1
  - Operating System: Ubuntu 18.04 LTS
  - Web Browser and Version:
  
  **What happened?**
  When trying to create a new user that already has its email taken by another user via the admin panel, nothing happens. Replicating the same result on the login page will at least throw up an error. Hopefully the same error message can be applied to the admin panel to prevent people from getting confused.
  
  ![image](https://user-images.githubusercontent.com/39305371/103143242-0f51b900-474d-11eb-8e37-87a85769ac8f.png)
  