Group import/export

Zendesk issue: https://gitlab.zendesk.com/agent/tickets/15717 Related to gitlab-org/gitlab-ce#3050

Allow a user to export all projects within a group. Bundle it as one downloadable archive. This depends on gitlab-org/gitlab-ce#3050 being complete.

This page may contain information related to upcoming products, features and functionality. It is important to note that the information presented is for informational purposes only, so please do not rely on the information for purchasing or planning purposes. Just like with all projects, the items mentioned on the page are subject to change or delay, and the development, release, and timing of any products, features, or functionality remain at the sole discretion of GitLab Inc.