Allow multiple soft credit contacts for a contribution

Allow back-office users to select multiple soft credit contacts for a given contribution. Users can select an existing contact or create a new contact (using the standard "select or create new" UI pattern as other objects).

Also allow users to specify the soft credit amount for each soft credit contact.

• Implement "select or create new" UI in add / edit contribution form.
• Link to "add another soft credit" inserts another set of soft credit fields for contact select/create.
• Each soft credit "row" contains contact select/create and soft credit amount.
• Soft credit amount field will be required, and will default to total amount of contribution.
• If user enters less than total amount of contribution AND adds another soft credit row, the soft credit amount default will be left empty (to allow for "variable splits")
• Users can not enter a soft credit amount that is greater than the total contribution amount (form rule)