Fork v5

This is an updated version of the previously rejected pull request: ![3145](https://github.com/gitlabhq/gitlabhq/pull/3145).

* "Fork" button on project page will create new project in user's namespace
* updated to use GitLab v5 rather than 4.2, including using gitlab-shell rather than gitolite
* improved test coverage
* addition of table forked_project_links to track which projects are forked from where
  * this will be important to enhance merge requests to go across forked repos