Update GET /v1/user API

[sergeychernyshev]

Update GET /v1/user API to return additional fields.

• email - string (optional - user’s email address if email is set)
• username - string (optional - user’s username if username is set)
• is_email_verified - boolean (is this email verified)
• impersonator - integer (id of the user who is currently impersonating this user - only return if user is impersonated)