Remove leading zeroes from proposal number

[gfrn]

Summary:

Since Matt's Shipping Service no longer requires it, proposal numbers are no longer padded with zeroes

Changes:

• Remove leading zeroes from proposal number

To test:

• Create a new shipment, add a dewar, puck, grid box and grid as normal, proceed to the final step where you can book a shipment
• Book a shipment (make sure you're pointing to the test environment of Matt's Shipping Service)
• Go to https://sample-shipping-test.diamond.ac.uk/shipment-requests and check if your shipment shows up there, without a leading zero in the proposal

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.68%. Comparing base (0aea769) to head (40751f6).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #29   +/-   ##
=======================================
  Coverage   94.68%   94.68%           
=======================================
  Files          42       42           
  Lines        1767     1767           
=======================================
  Hits         1673     1673           
  Misses         94       94           

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