Fix missing HUD item for Lua

So, toggleable HUD elements weren't working as intended because of a missing entry in HUD element table, which made coopemeralds disable rankings, for example. Not only does this patch add this missing entry, and thus fix the problem, but I'm also introducing I_StaticAssert which allows us to check these things at compile time. This should hopefully get rid of this bug vector entirely over time, as we'll get a compilation error if related tables are out of sync.