Enable posting comments by student on submitted tasks

[messa]

Until now only coaches could send comments (on submitted tasks) and only after that the student could reply to these comments.

cc @mintaka

[Mintaka]

And also, sometimes we need put comment to task, where student even don´t send solution. For example in case when we talk about that task with this student elsewere and want to send her sample solution.

So student or coach should comment any time. If there is no "version 1" than attach it to "version 0" for example.