Allow administers to enter custom amount when submitting a credit card payment for membership

Currently various contribution amounts can be configured on profile pages but when submitting a credit card payment for membership it does not permit an amount to be entered and assumes the minimum_fee associated with the membership. Patch allows the back-end user to enter the amount in. If the field is left blank then it defaults to the calculated amount (similar behaviour to start & end dates).

Patch also assigns contributionID to template. It assigns 'amount' if the payment was a success. Since the presence of 'amount' as a field is a conditional used in the smarty template I didn't assign it outside of the scenario of a successful credit card payment.