Implement "Sales" when a resource is native to that planet

[evangipson]

For instance, you are on planet XYZ and they have resource ABC as a native resource - so when you are there it's 50% the cost.

Right now I'm just using "cost" for the inventory and math... so I'm going to need to write like a "buyWithSale" or something which will take in a saleAmount then handle all the math that way so the graphs will remain correct (I don't want to mess with the actual cost - or perhaps a "buyStock(salePrice)" function which will take an argument that could be a sale price?)

Dependencies

• Issue Add "Planet Info" button in controls #34
• Issue Implement real-time notifications #39