Implement negotiation for salvage terms

[Algebro7]

This adds the ability for Admin/Logistics personnel to reroll salvage terms when negotiating AtB contracts. The plumbing was already in place--I mostly just had to add the UI elements.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 9.89%. Comparing base (cef4c6c) to head (8d78bff).
Report is 55 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #4641      +/-   ##
============================================
- Coverage      9.90%    9.89%   -0.01%     
  Complexity     5676     5676              
============================================
  Files           945      945              
  Lines        129636   129663      +27     
  Branches      19092    19098       +6     
============================================
  Hits          12836    12836              
- Misses       115583   115610      +27     
  Partials       1217     1217              

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[IllianiCBT]

Once the tests pass, this is good to go.