Atomic operations on handling submission repos

[ksurya]

If two users are simultaneously editing a submission, the current system can't provide a mechanism to correctly handle the situation.

There is a possibility of happening the below

1. While one user starts adding changes, another user also starts adding. Whereby anyone user commits changes which itself is wrong

[ksurya]

One way to solve this is to use django.contrib.auth.models.Permission to have permissions like can_edit which is linked to scipy_central.submission.models.Submission. Storage dir should only be accessed when can_edit is True.