redirect to home page when register rather than redirect to cas login

useful for the case of manual account validation

TODO: consider patching register_by_email_activation also
reverse maintain_standard_login feature implementation

from usability perspective, it's clearer to have "CAS connection" as a add-on rather than replace the default when maintain_standard_login is set.
now CAS connection is the second option to login and not the first when maintain_standard_login is set.

account menu before:
  (Sign In => CAS) (Standard Sign In => Login_Form)
account menu after :
  (Sign In => Login_Form) (CAS Sign In => CAS)
extend login form with a CAS Sign in link

use case : enable CAS Sign in and deep link inside redmine, otherwise user wouldn't be redirected to back_url.