Fix global ignore variable

[frances-h]

Globally ignores a variable (including preventing Identity Feature from being created) if global options do not conflict with any primitive options

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After creating the pull request: in order to pass the changelog_updated check you will need to update the "Future Release" section of docs/source/changelog.rst to include this pull request.

[codecov-io]

Codecov Report

Merging #819 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #819      +/-   ##
==========================================
+ Coverage   98.12%   98.12%   +<.01%     
==========================================
  Files         119      119              
  Lines       10877    10883       +6     
==========================================
+ Hits        10673    10679       +6     
  Misses        204      204

Impacted Files	Coverage Δ
featuretools/primitives/options_utils.py	100% <100%> (ø)	⬆️
featuretools/synthesis/deep_feature_synthesis.py	97.02% <100%> (+0.01%)	⬆️
...ols/tests/synthesis/test_deep_feature_synthesis.py	99.07% <100%> (ø)	⬆️

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[rwedge]

Looks good