Don't use magic numbers for ChangeTypes.
Start extracting data corrections into their own objects.

Changes to amount, facility_letter_date, initial_draw_date and initial_draw_amount have been removed and are no longer allowed.
Separate class for correcting demanded amount and interest.
Data correction breadcrumbs.
Merge pull request #137 from alphagov/data-corrections-refactor

Simplify data correction logic and flow by separating each change into its own class