Git forks now in public preview

Introduces Git forks as a public preview feature in VSTS and TFS 2018 RC1. Forks are complete, efficiently stored copies of repositories used for isolating changes. They support pull requests for merging back to the original repository and can be managed at the per-repository or account level. Initial limitations include forking only within a single VSTS account, and the requirement of 'Create repository' permission for fork creation.