Units defending on a River tile should gain a combat bonus.

Currently, all units fighting on rivers gain a "river crossing" debuff that lowers there combat strength. Historically, this makes sense, but only for the armies that were attacking (mostly). Armies that were entrenched on the opposite side of the river had an enormous defensive advantage. This should be represented in-game with a defensive buff to the defending army that is stationed on a river tile, while the attacking army should still receive the offensive debuff for crossing the river to attack.