Add created_at field to Reaction

[hoenn]

This pull request exposes the created_at field for reactions.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.21%. Comparing base (0877faf) to head (040af46).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3501   +/-   ##
=======================================
  Coverage   91.21%   91.21%           
=======================================
  Files         182      182           
  Lines       15930    15930           
=======================================
  Hits        14531    14531           
  Misses       1225     1225           
  Partials      174      174           

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[gmlewis]

Thank you, @hoenn!
LGTM.
Merging.