self-fork work-around should not be part of RCloud core

[s-u]

Now that we have a clean separation of notebook back-ends and RCloud, the limitations of some back-ends should be worked around in the backend interface and not in RCloud. Prominent example is forking of your own notebooks which is supported in the gitgist back-end directly but needs a work-around in the githubgist backend.

[gordonwoodhull]

It is indeed annoying to have these workaround at the UI level, but I think it will take some effort to untangle them. Pushing to 1.3