LIMS-2068: Dont allow duplicate requests to shipping service

[ndg63276]

JIRA ticket: LIMS-2068

Summary:

If a user doesn't complete the shipping service request, and uses the browser back button to return to Synchweb, the "Proceed" button can create duplicate shipping requests, which cause errors.

Changes:

• On the front end, check for an existing shipping service request, and take you there if you click Proceed again
• On the back end, throw an error if something gets past the front end check

To test:

• This PR has been deployed to ispyb-dev-1 to aid testing, although it seems to struggle to authenticate with sample-shipping-test for some reason. Needs to be tested from a *.diamond.ac.uk address.
• Create a shipment, create an AWB through the shipping service. Check it all works as expected and takes you back to Synchweb
• Check the "Create DHL Air Waybill" button has become "Manage Shipment Booking", and this still takes you to the shipping service
• Go to the shipment in Synchweb, but edit the URL from /shipments/sid/nnnnn to /shipments/awb/sid/nnnnn. This should take you to the "Create DHL Air Waybill" page again.
• Tick a dewar and click Proceed. Check you are taken to manage the booking on the shipping service, not creating a new booking.
• Extra bonus points - take out the front end check and repeat the last test, check you are blocked from creating a new shipment request.