Allow importing exercises from a git repository

[yammesicka]

Be sure to create async request.

We should research the option to add lms to git remote.

[gal432]

@yammesicka WDYT?

On the Exercises view, next to the Send and the notebook number, we will add a new button that will copy to the clipboard the URL that the student can push the exercise to (We can add a new modal with git instructions.

If the student will add this remote and will run a git push, a new Solution will be created with the same logic as any notebook/zip upload for the main branch.
In case of multiple branches in the repository / different branch name - we will dismiss the push.
The URL will be allowed to git operation only for this specific user and for open exercises.

for the first version, we will implement the service for HTTP(s) based git operations, with implementing a stub remote git backend that will allow users to push local branches. In the future, we can use this for making a template repository structure for exercises.

Docs

• https://git-scm.com/docs/git-http-backend
• https://gist.github.com/stewartpark/1b079dc0481c6213def9