Role-based balancing

[UncleVasya]

• [x] Create RolePreference model; 1-to-1 with Player.
• [x] Commands to set preferecne:
  • [x] !role pos2 4;
  • [x] !role mid 4;
  • [x] !role offlane 5;
  • [x] !role off 5;
  • [x] !role carry 2;
• [x] balance tries to start with 2 midlaners instead of 2 top mmrs;

[UncleVasya]

Smart role-balancing:

• [x] get all combinations of team roles;
• [x] assign to every balance the best combination of player roles, by pref score sum;
• [x] sort balances by pref score sum;
• [ ] sanity filters to check that teams are competitive;

[UncleVasya]

Sanity checks will be done in #57