Add total amount to the fees

This is for the front page of the PDF form, where the
'Total amount' is calculated once the solicitor's fee
is known.
PDF Form field renamed for total cost

When the court fee and solicitor's fee are known, they are added
and filled in into the PDF form
Claim spec for total_cost revised

Made the intent clear for the situation when the solicitor's fee
is not known.
Merge pull request #62 from ministryofjustice/66656806/calculate-total-amount-for-court-fee

66656806/calculate total amount for court fee