Add mini-css-extract-plugin to app/react dependencies

[chadfawcett]

Issue: Related to #4405 and failing tests of #4524

What I did

Added mini-css-extract-plugin to app/react dependencies

How to test

Automated tests should cover this

[codecov]

Codecov Report

Merging #4534 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master    #4534   +/-   ##
=======================================
  Coverage   35.58%   35.58%           
=======================================
  Files         557      557           
  Lines        6730     6730           
  Branches      883      883           
=======================================
  Hits         2395     2395           
  Misses       3876     3876           
  Partials      459      459

Impacted Files	Coverage Δ
app/react/src/server/cra_config.js	0% <0%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 182bccc...ab85679. Read the comment docs.