Auto provisioning and de-provisioning of users

GitLab should explore the idea of user authentication and management, specifically the idea auto provisioning and de-provisioning of users, as a Premium (or greater Edition feature. Many enterprise companies are driving, both from a security and user management stand point to a need to manage access to their tools seamlessly and automatically. The first step in this might be leveraging LDAP Group Sync specifically as an enterprise piece of functionality via Premium. @mrogge @jmay1