Storing courses and tasks in Git repositories

[nrybowski]

This PR lays down the basics for #1059 and related features. For more details, see #1059.

TODO list

Git-based courses and tasks

• [x] Implement the FileSystemProvider interface for the Git-based FS. > Added with f5bf65c1b329c8070d9d12e0110e2b656b0a1259.
• [x] Git FSP-based courses. > Added with 93d996b84a24d00ff70c9203285d0176e6590d0f.
• [x] Git FSP-based tasks.
• [ ] Initialize sub-module upon task creation.
• [x] Abstract Gitolite interaction for course or task creation on remote.
• [x] Remote configuration through INGInious config file.
• [ ] Handling $common directory.
  • [x] Upon $common directory creation
    • [x] Create course-scoped sub-module. ~- [ ] For each task, create a task-scoped sub-module.~
  • [ ] Upon $common directory update:
    • [ ] Commit changes in the $common directory.
    • [ ] For each task, bump $common sub-module version.
    • [ ] Bump the course-scoped $common sub-module version.
  • [x] Upon task addition, add the $common sub-module.
• [ ] Version task edition and update course submodule reference.
  • [x] Creation
  • [x] Edition
  • [ ] Deletion
  • [x] Upload
  • [ ] Rename ~- [ ] Handling $i18n directory?~ ~- [ ] Automated signed commit upon task / course modification at the web GUI.~

[nrybowski]

Open question: How do we handle task migration with common directories?

Suppose we have courseA/task0 and courseB. The first course contains a $common directory: courseA/$common and courseA/task0/$common whose remote origin points towards remote:courseA/$common. courseB does not contain a $common directory.

We migrate courseA/task0 to courseB by adding a courseB/task0 sub-module whose remote still points to remote:courseA/$common. As courseB does not contain a $common directory, we can add courseA/$common submodule to courseB and update remotes from courseB/$common and courseB/task0/$common to remote:courseB/$common.

Now suppose that courseB already contains an unrelated $common sub-module. How do we merge courseA/$common with courseB/$common?

[nrybowski]

With this push, we moved away from GitPython to leverage pygit2 instead. See https://github.com/INGInious/INGInious/pull/1058#discussion_r2243285240.

[nrybowski]

After discussing with @anthonygego and @AlexandreDoneux, this PR will remain on hold until the course and task factories are correctly refactored.