Support Fork method

[afeld]

As described in the guide, using forks for assignments is preferable for a lot of reasons. Would be great to have the following actions support forks of assignment repositories:

• open_issue
• clone_repos

Note: will probably want to filter by current members of the students team a list of students or a particular team, so that it doesn't grab repositories from past iterations of the class.

[tarebyte]

Is this one already completed? I see both an open_issue action/command and a clone_repos action/command.

[afeld]

Nope, still open. I just meant that those actions work for the Sandbox strategy, but not for Forks.