[backend logic] Subsidy/reputation framework ground work

[jpcsupplies]

For the transport tycoon subsidies, and reputation modifiers.. we may need to add a "reputation" field on the bank ledger.
This could later be used for mission systems, for example preventing players with a bad reputation getting delivery/escort missions.

Likewise in the buy/sell code we need to add a modifier of some sort so that it checks for subsidies and recalculates prices accordingly if one is present.