Upgrade Storybook to latest v5

[acusti]

note that this branch was branched off of #152 and will be easier to review in isolation from those changes.

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improves the UI/UX of the docs. i tried upgrading to v6 but there were too many breaking changes and deprecated plugins to feel worthwhile.

[codecov[bot]]

Codecov Report

Merging #153 (4ba749a) into master (2599c5d) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #153   +/-   ##
=======================================
  Coverage   96.55%   96.55%           
=======================================
  Files           2        2           
  Lines         145      145           
  Branches       41       41           
=======================================
  Hits          140      140           
  Misses          5        5           

Impacted Files	Coverage Δ
src/ReactCodeInput.js
src/utils.js
ReactCodeInput.js	96.45% <0.00%> (ø)
utils.js	100.00% <0.00%> (ø)

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