Streamlined User Management

Introduces a preview of a new account-level user hub experience in Visual Studio Team Services (VSTS). This feature simplifies user management by allowing administrators to invite users with proper extensions, access levels, and group memberships in a single step. It also provides a re-designed Users page with an 'Extensions' column for at-a-glance understanding of user access, and a detailed view for individual users to manage their access level, extensions, and group memberships.