Don't overwrite local code when saved submission on server differs

[heyzec]

When student clicks on a mission, quest or any other assignment via the SA panel in the extension, the extension will take the server's copy and save it locally. This is a problem if the student continued editing on their local copy of the assignment while offline and changes were not saved.

We need to detect when local and server copies of the submission differs. If so, we need some way to prompt the student and ask which copy they would like to use.

[heyzec]

Turns out the premise in the issue is false - currently the extension will not fetch updated code from server (e.g. student edited code on their browser). Implement this update logic first before tackling the issue.

[heyzec]

Fixed by #21