updated fork feature to use gitlab-shell for v5 of gitlab

Created by: amacarthur

Replaces pull request https://github.com/gitlabhq/gitlabhq/pull/3382

Dependent upon gitlab-shell pull request: https://github.com/gitlabhq/gitlab-shell/pull/45

This will provide a button on the project page which will create a fork of a project into their own namespace. The fork from/to relationship is recorded in anticipation of gitlab handling cross-repo pull requests.