Fix DeveloperSignup form reload, wrong error messages, password visibility

[thessakranendonk]

• The DeveloperSignup form reloads after submit and therefore the form shows error messages briefly or not at all.
• No error message if username already exists, skips right to email validation.
• Error message for a short password displays error message at email input ("Email already exists") instead of password.
• No error message if password does not match confirmation password.
• Add password visibility toggle.