Use ReactDOM webpack external

[mickmister]

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Summary

This plugin currently doesn't use react-dom, but I'm adding this here in case we add a library like react-select in the future.

Ticket Link

Fixes https://mattermost.atlassian.net/browse/MM-47046

[codecov[bot]]

Codecov Report

Base: 28.48% // Head: 28.48% // No change to project coverage :thumbsup:

Coverage data is based on head (b0be6b6) compared to base (2c3003b). Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #318   +/-   ##
=======================================
  Coverage   28.48%   28.48%           
=======================================
  Files          20       20           
  Lines        2619     2619           
=======================================
  Hits          746      746           
  Misses       1789     1789           
  Partials       84       84           

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