text: Title: The resend_activation endpoint discloses if a user with a given email exists
  Body: The `resend_activation` endpoint returns a 400 response if the given email does not belong to an (inactive) user.
  
  This endpoint re-uses the password reset serializer (#555) but does not respect the `PASSWORD_RESET_SHOW_EMAIL_NOT_FOUND` setting because of these lines:
  
  https://github.com/sunscrapers/djoser/blob/c62371e3f9a8bbad2eaf55ffd0efad6eb6c02f26/djoser/views.py#L208-L209
  
  All settings related to disclosing email default to `False`:
  
  ```
  PASSWORD_RESET_SHOW_EMAIL_NOT_FOUND
  USERNAME_RESET_SHOW_EMAIL_NOT_FOUND
  ```
  
  `resend_activation` shouldn't break this default.
  
  P.S.
  
  These settings don't work as advertised by the way, setting one to True has the effect of also toggling the other: 
  https://github.com/sunscrapers/djoser/blob/c62371e3f9a8bbad2eaf55ffd0efad6eb6c02f26/djoser/serializers.py#L145-L149
  