Display plague risk in SDL clients
Plague risk isn't currently visible anywhere in the SDL clients. Add it to the "city info" tab of the city dialog, alongside pollution.
I haven't added an icon for it (unlike pollution), as I don't have one to hand and there isn't really room with trade routes as well.
(I've removed a massive gap between pollution icons and trade routes, to make room. I'm not sure why this was there; it's been there since the code was first added; I suspect a typo.)