Registration: Do no expose email is taken, instead send "account exists" notice

[Brutus5000]

Currently the registration form reveals if an account for a certain email exists or not. This contradicts our security measure on password reset where we do not reveal if an email is in use.

So instead of telling the user, that the email is taken instead we should do the following:

• Send an email on to the given email address
• Put in the actual username and the desired username
• Mention the "no double account rules"
• Contain a link to email password recovery (with prepopulated email address - new feature)
• Contain a link to rename account feature

[Ivan-Shaml]

Break down into smaller tasks:

• Refactor Registration Service, to send email on email taken
• Define email template text with variables

Sub features:

• [feat] Expose path /recover-account/email?identifier=${email/username} (TBD)
• [feat] Create change username

[Brutus5000]

Also hint: The email source code is here https://github.com/FAForever/faf-user-service/tree/master/src/main/mjml

[Ivan-Shaml]

email template current state: