Integrate Flow typing

[vzhou842]

Description

Integrates Flow.

• Installs gatsby-plugin-flow, the official Gatsby plugin for Flow
• Installs flow-bin and @babel/plugin-transform-flow-strip-types as dev dependencies
• Adds flow type check to the lint script
• Adds '@babel/plugin-transform-flow-strip-types' to jest-preprocess so Jest tests still work
• Updates README
• Adds /src/flow/CSSModuleStub.js so that Flow ignores CSS module imports. This is the standard way to do this.
• Adds Flow typing to Sidebar.js and index-template.js as an initial test to show that Flow works.

Related Issues

Discussed in #176.

[codecov]

Codecov Report

Merging #178 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master    #178   +/-   ##
======================================
  Coverage    95.1%   95.1%           
======================================
  Files          28      28           
  Lines         143     143           
  Branches        8       8           
======================================
  Hits          136     136           
  Misses          7       7

Impacted Files	Coverage Δ
src/templates/index-template.js	100% <100%> (ø)	⬆️
src/components/Sidebar/Sidebar.js	83.33% <100%> (ø)	⬆️

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[alxshelepenok]

@vzhou842 thanks for the contribution!