perf(Flag|Icon): add shouldComponentUpdate with shallowEqual

[layershifter]

Rel #1020.

I've imported shallowEqual in #1981 and remembered about #1020. Converts Flag and Icon to class components and adds shouldComponentUpdate with shallowEqual.

[codecov-io]

Codecov Report

Merging #2011 into master will decrease coverage by 0.03%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master    #2011      +/-   ##
==========================================
- Coverage    99.8%   99.76%   -0.04%     
==========================================
  Files         148      148              
  Lines        2561     2556       -5     
==========================================
- Hits         2556     2550       -6     
- Misses          5        6       +1

Impacted Files	Coverage Δ
src/elements/Icon/Icon.js	100% <100%> (ø)	⬆️
src/elements/Flag/Flag.js	87.5% <83.33%> (-12.5%)	⬇️

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[levithomason]

Released in semantic-ui-react@0.73.0