[RUN AS-IF-FOSS] Move saas_user_caps feature flag definition to CE

We reference the saas_user_caps feature flag inside app/views/groups/settings/_permissions.html.haml even though it is a EE feature. To avoid CE failures, move this feature flag into the CE directory.

Sample failure: https://gitlab.com/gitlab-org/gitlab-foss/-/jobs/1538073660