- use login_ajax (in place of login_session with shim) for
logistration's call to login POST
- add toggle for using login_ajax from logistration
- FEATURES['ENABLE_LOGIN_POST_WITHOUT_SHIM']
- add custom metrics for redirect_url
- update test for third-party auth error_code
NOTE: The error_code `third-party-auth-with-no-linked-account`
was introduced in JSON in this earlier PR:
https://github.com/edx/edx-platform/pull/22452/files
ARCH-1253