Prepare fork to given namespace

Issue #210 (closed) reported that fork into a group's namespace is not possible thru gitlab and in the merge request "to be able to fork a project through the API", two people wanted to also be able to fork to a group's namespace.

Example user story for this from the fork merge request: "+1 For training courses we are running, we want to fork a project in a known state, rename it and then move it to a Training Group. It would be great to be able to script this."

This is the 1st of two commits to make this happen:

Add the Middleware for supporting forks to a given namespace. gitlab-shell's 2nd argument for fork is already the namespace in which the forked project shall be created. This commit makes this functionality available in the gitlab-rails layer.