SAML uses inconsistent redirect

Zendesk issue: https://gitlab.zendesk.com/agent/tickets/14332

If a user is not already authenticated with the SAML server and they click to login to GitLab, they are authenticated and then returned to the GitLab server at the root/dashboard page. However, if a user is already authenticated with SAML and they sign in to GitLab they are redirected to '/profile/account' instead of the dashboard. Users should always be redirected to the dashboard after sign in.

cc/ @patricio Will you please help me test this?