Add UI for shipment form

• Editing and creating, but I changed the routes a bit: there's now a single /shipments route, and you pass query params to render the form: /shipments?new and /shipments?edit=shipment-id.
• Two fields in the form for now: invetigationId and name.
• No back-end fetch/mutation -- everything is mocked.
• Type e in the name field to get an error and see the error message on the form itself (instead of going back to the list of shipments).
• Type anything else in the name field to go back to the list and see the success message.