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.