When a user without a SSO session attempted to access anything in a SAML
group, previously GitLab would redirect the user back to the dashboard
and lose the original link. This was happening because the `RelayState`
wasn't being used since the `SessionsController#create` took over. To
fix this, we have to do things:

1. Store the `RelayState` in the Devise session helper before the
session is created. `ApplicationController#after_sign_in_path_for` will
run and retrieve this value.

2. Ensure the `RelayState` is the right value. Previously this value
would be set to the path of the project (e.g. mygroup/project), but this
drops the full path that was being accessed. Now we retain the entire
path so the `redirect` parameter is passed on to the IdP properly.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/247674

Changelog: fixed
EE: true