Hide C-Side/Golden postcard if LevelSet has no C-Sides

[coloursofnoise]

• Fixes C-Side + Goldenberry postcard should have a toggle or automatically be disabled if the map pack does not contain C sides #357

Relatively simple hack to only show the C-Side Unlock Postcard if the level set has a C-Side.

Adds a public LevelSetStats.MaxAreaMode property that returns an int representation of the highest available AreaMode for the current level set.
Returning an int makes for a slightly simpler IL patch, and hopefully promotes compatibility with some future integration of the AltSidesHelper mod into Everest. This can easily be changed to return an AreaMode if desired.