Allow preview from forks

[osrf-migration]

Original report (archived issue) by Louise Poubel (Bitbucket: chapulina, GitHub: chapulina).

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It would be helpful for external contributors if they could preview their tutorials.

Consider adding &fork=fork_name to the URLs.

[osrf-migration]

Original comment by Steve Peters (Bitbucket: Steven Peters, GitHub: scpeters).

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Alternatively, it could be owner instead of fork. Fork is shorter though

[osrf-migration]

Original comment by Louise Poubel (Bitbucket: chapulina, GitHub: chapulina).

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Yeah true, fork sounds like the fork name, which in most cases is the same as the original repo.