Allow making upgrades for both cars same week

[Bimbalas]

Each car has 5 mechanics, so I see no sense that only one car can be upgraded. Training only one mechanic is still OK and requires strategy from team manager which one to train. Finally, mechanics level should be the deciding factor, not the number of upgrades per team.

[giedriusr]

Something like this is planned. We're brainstorming on a new service park system.