Allow the teacher to edit students answers

[yurug]

Currently, students' answers are read-only. It would be handy for a teacher to be able to annotate them to give some feedback about the code.

[AltGr]

Allowing the teacher to edit the solution would be fairly simple by itself, but the difficulty will come from concurrent editing: we'll need either to implement a signalling mechanism for synchronous edition (completely doable, but quite out of scope of our current server implementation) ; or a reliable two-way merge upon synchronisation (it's at the moment push / pull only — to be fair this can already be an issue for students logging in simultaneously from multiple machines using the same token)