Avoid user enumeration by redirecting duplicated requests

[adrianiftode]

When an account already exists the user is redirected to the registration success screen as there is no error, in order to avoid user enumeration

What does it fix?

Closes #440

Please mention the main changes this PR brings.

• redirects the user on DuplicateUserName error
• the email is not chcked anymore on the registration success screen
• EmailConfirmation is not configurable anymore in order to avoid different behaviors on the registration screen

How has it been tested?

Manually tested it

• register abc@mailinator.com
• confirm email
• confirm login works
• register again with abc@mailinator.com
• this PR behavior: the user is redirected to the registration confirmation screen as the first time
• original behavior: the user was present an error message informing him/her and account already exists

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