Updated Primary Allies Scenario Modifiers

[IllianiCBT]

The two primary allies modifiers are a special case that is randomly added to a scenario, based on the contract command level (or contract type)...

...this runs contrary to the modifiers' description which made it seem like mhq was reacting to the size of enemy forces.

Instead, this modifier would increase the size of the OpFor (by increasing the size of the player-allied force), resulting in the 'overwhelming enemy forces' problem it was claiming to 'solve'.

This PR restores the ability for these modifiers to contribute to BV (that had been accidentally dropped some time around 50.0) and updates the modifier description. While the original description was well intentioned, and technically accurate, it contributed to a lot of confusion over the months whenever Force Generation and balancing were being discussed.

Closes #5008

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 10.41%. Comparing base (a339462) to head (52bc6e7).
Report is 113 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5074      +/-   ##
============================================
- Coverage     10.42%   10.41%   -0.01%     
+ Complexity     6028     6025       -3     
============================================
  Files           953      953              
  Lines        134113   134121       +8     
  Branches      19435    19437       +2     
============================================
- Hits          13979    13974       -5     
- Misses       118785   118801      +16     
+ Partials       1349     1346       -3     

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