Store the conversion rate used for state aid calculations until now.
Save the euro_conversion_rate when the StateAidCalculation is created.
Use the latest exchange rate when initially scheduling a loan.

Before we have guaranteed the loan we always use the
latest exchange rate to calculate the state aid calculation.
Use a method instead of a constant to allow a superuser to change this value (when we develop this in the future).
Tidy StateAidCalulcation constants.
Change Euro exchange rate.
Merge pull request #114 from alphagov/change_euro_conversion_rate

Change Euro Conversion Rate