[WIP] Make nicer Braintree account form

- [ ] Some naive front-end validations, e.g. account and routing numbers present
- [ ] Change the input field sizes so that the size on an input indicates the expected input data length (i.e. finetune input fields and their sizes)
- [ ] Dropdown for state
- [ ] Date of birth
- [ ] Tests that front end validation errors are shown (and also that Braintree validation errors are shown) (not tested yet due credentials needed)