Add an option to re-enable AI city trading

[LStuber]

As a result of (6-60) Can't Sell Cities Founded By Their Current Owner, AI city trading is now disabled.

However, this change has been hardcoded in the DLL. It would be great to make it optional instead, especially since the proposal was accepted only by a small margin (46% voted Nay).

Side-note: I would suggest to make this option editable only in the config files first (sql), but in the future I'll probably make a proposal to have this as a main setup option, as well as several of the other AI restrictions, under a single option "Make AI less exploitable" as I believe all these changes are too obscure for new players.