Add "school year" as a field in the contest model

[Kostero]

By default it should auto-fill to the current school year.

I believe it should also be displayed in the list of contests, or even better: the contests should be grouped by the school year.

I believe this will also be very helpful in Szkopuł, where teachers rarely use the same contests between different school years.

[segir187]

While it is true that contests are rarely used between different school years, I believe there should be an ability to choose the starting and ending school years, with the latter being possibly blank (ie. the contest is ongoing and we don't know how long it's going to keep going).

Either that or just make the field completely optional, though that I believe might cause an issue with motivation for teachers to fill it in - in that case, maybe force the contest creator to either select the school year or "Not applicable".