Re-enable trade-in of ships.

# Description:
Y'know what they say: It halves in value as soon as you drive off it the forecourt!
Well the same applies to ships :)
You can trade in your old ship, at 50% of the value, for a shiny shiny new one once more.
If you buy a cheaper ship you'll get money back, if you get a new one... well you'll have to fork out for it.

# Only Commit:
Subtract 50% of the price of the players ship from the cost of the new one.
Refund the difference.

Fixes issues: #2343 & #2087