Implemented payment form for bank transfers

Finished implementing the payment form for bank transfer payments. Currently there is no interface to see payments, so you'll need to look in the database to verify that it's working.

Changes:
- Server now saves payments created using the form
- Client generates a unique bank transfer ID to be used for reconciliation
- Client & Server both validate form input
- New fields in the database - bank_reference for reference for bank transfers, and pending to mark payments not yet recieved