[Pending review and testing] Organization's payment information

- [x] Write Cucumber test
- [x] Move merchant key and id and company_id from organization to person
- [x] Store the data (required for checkout account) from the form at payments tab to person
- [x] Create Checkout account and store received merchant id (when details form filled)
- [x] Make payments tab visible when checkout is in use as payment gatweway
- [x] Show Checkout registration help text
- [x] Show message about existing Checkout account, if user has already filled payment information
- [x] Add client-side validations
- [x] Add server-side validations
- [x] When paying, use information from new columns in person table