Feature: 1A style group numbers for separate zones competitions

[AlexKatyukov]

Let me explain.

1. At YJ Fest 2018 we have 3 zones: two (named A and B) at first floor and one (named C) at second floor. We divided competitors to groups named 1A, 1B, 1C; 2A, 2B, 2C; 3A, 3B, 3C etc. It was very convenient.
2. At YJ Fest Moscow 2019 there was similar setup but groups was named just with number: 1, 2, 3, 4, 5... Competitors with group numbers 1, 4 and 7 competed at zone A, competitors from group 2, 5 and 8 should go to zone B, and 3, 6, 9 group competitors should compete in zone C.
3. Now at SPB Muffin 2020 we have similar issue. Luckily we have only 2 zones for all events and we can say "odd-number groups here, even-number groups here". But in my opinion 1A-style group numbers like in 1 is more comfortable to competitors and for organizers.