Add Project Import/Export Functionality

• Zendesk ticket: https://gitlab.zendesk.com/agent/tickets/9876
• Feedback forum link: http://feedback.gitlab.com/forums/176466-general/suggestions/4488044-add-import-export-functionality-for-projects

Add the ability to export/import single projects. This feature has a very large number of votes on the feedback forum. A customer also requests this feature. This will make it very easy to migrate single projects either from GitLab.com to a self-hosted instance or between self-hosted instances.

Next steps

We're currently 'accepting merge requests' for this. Due to the large interest, including from a customer, can we investigate this a little more and put it on the roadmap?

/cc @JobV