Use JSON endpoint for reading goal partials

[jimchamp]

Closes #7447

Changes YRG partials endpoint to use JSON encoding.

This endpoint is called whenever a patron sets a yearly reading goal from their /account/books page.

Technical

Testing

Screenshot

Stakeholders

[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (5643463) 16.68% compared to head (2cc5f78) 16.68%.
Report is 19 commits behind head on master.

Files	Patch %	Lines
...nlibrary/plugins/openlibrary/js/check-ins/index.js	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8662      +/-   ##
==========================================
- Coverage   16.68%   16.68%   -0.01%     
==========================================
  Files          88       88              
  Lines        4681     4682       +1     
  Branches      835      835              
==========================================
  Hits          781      781              
- Misses       3384     3385       +1     
  Partials      516      516              

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[mekarpeles]

lgtm!