Add Montana taxable social security benefits

[hua7450]

Create a variable for Montana taxable social security benefits. (This variable can be used on either addition or subtraction depending on its amount)

[hua7450]

The parameters in this PR only appears in the tax form.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.99%. Comparing base (f098a60) to head (70fbb46). Report is 3 commits behind head on master.

:exclamation: Current head 70fbb46 differs from pull request most recent head 0c9aa50

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4510   +/-   ##
=======================================
  Coverage   98.99%   98.99%           
=======================================
  Files        2450     2454    +4     
  Lines       35607    35679   +72     
  Branches      184      184           
=======================================
+ Hits        35250    35322   +72     
  Misses        327      327           
  Partials       30       30           

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