Add Policy.JSON_REFORM_SUFFIXES set and related test

[martinholmer]

This pull request simplifies the col_label names in the current_law_policy.json file.
The simplifications include making them all lower case, eliminating spaces, and shortening long labels.

The purpose of this pull request is to prepare for the addition of an optional capability that will simplify the specification of JSON reform files when the reform includes changes in policy parameters that are indexed (by MARS, EIC or idedtype). That optional capability is primarily intended for the use of TaxBrain --- see TaxBrain issue 596 --- but may also be used in hand-prepared JSON reform files. This new capability will be added in another pull request.

I have looked at the TaxBrain code in the webapp/apps/taxbrain directory and have not found any code that would be affected by the changes in col_label values in current_law_policy.json. The TaxBrain code does check for the EIC-indexed col_label names, but those are unchanged in this pull request. So, as best as I can tell, the changes in this pull request do not change the Tax-Calculator public API.

@MattHJensen @feenberg @Amy-Xu @andersonfrailey @hdoupe @GoFroggyRun @codykallen
@talumbau @PeterDSteinberg @brittainhard

[codecov-io]

Codecov Report

Merging #1503 into master will decrease coverage by 0.04%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #1503      +/-   ##
==========================================
- Coverage   99.95%   99.91%   -0.05%     
==========================================
  Files          37       37              
  Lines        2481     2484       +3     
==========================================
+ Hits         2480     2482       +2     
- Misses          1        2       +1

Impacted Files	Coverage Δ
taxcalc/policy.py	98.43% <66.66%> (-1.57%)	⬇️

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[martinholmer]

The one additional untested statement in this pull request is just a placeholder for code that will be added in a future pull request to the Policy.translate_json_reform_suffixes() static method.