Refactor assignment model
This merge request is part of !166 (merged)
- New argument
initializedforProject.ensure_forkandAssignment.ensure_submission_repoto ensure that the project should be initialized with the content of the origin repository upon creation -
GitLab.get_branch: raiseResourceNotFoundif the branch is missing (minor backward incompatibility) - Simplify the student group / leader logic to make it more stateless (to be pursued in a follow up MR)