State Aid is added at completion.
Don't recalculate state aid every time a premium schedule is created.
A newly created premium schedule has the same duration as its loan.
Move state aid calculation out of the premium schedule.

Prepare for imminent changes in the calculating of state aid by moving
the current logic into a phase-5-specific class.
Move logic out of the controller.
Merge pull request #200 from alphagov/state-aid-refactor

Move state aid calculation out of the premium schedule