Add option to use Roster ID for Assignment-Repo suffix

[ReshefElisha]

What

What does your pull request propose? ✨
This PR implements a way to use the existing roster to organize student assignment-repos. A checkbox is added to assignment creation enabling the instructor to specify the use of the student's roster-id instead of their github-username as assignment-repo suffix.

This applies to #630 and partially to #1626

Heads up

app/views/assignment_invitations/show.html.erb still uses "#{current_assignment.slug}-#{current_user.github_user.login}" for the displayed repo name. This has no bearing on functionality but may confuse users as the final repo name will not be the one displayed. I'm unfamiliar with RoR so I'm not sure what the visibility of the generate_github_repository_name function is to the view. If possible, it should be used anywhere the repo-name needs to be used or shown. Please advise how to proceed.

[ReshefElisha]

re: the assignment_invitation issue: It's also inaccurate in case a repo needs a suffix. This is a larger scope so may warrant a different PR? Currently in that view the AssignmentRepo current_submission is nil, until the user chooses to accept the invitation. This may mean the generate_github_repo_name method needs to be reimplemented in the assignment_invitation controller to remove depending on having a current_submission.

[BenEmdon]

Thanks for your contribution, I agree that this is good idea and I want to see this happen.

Notes

• Your commit history seems to overwrite some commits I made:
  
  You would have to clean that up for us to consider this PR.
• Please separate the schema migration into a separate PR so we can run the migration with 0 downtime.
• You need to add a test to spec/models/assignment_repo/creator_spec.rb to test this new functionality.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. Thank you for your contributions.