[WIP] Changes made during Peril work

[orta]

• Make the GitHub API not rely on a CI instance, but on an interface which it cares about
• Adds more typings from Jest
• Adds more fixtures
• Updates babel deps
• Updates jest dependencies Updated Jest devDependencies #198

[codecov-io]

Codecov Report

Merging #194 into master will decrease coverage by 0.08%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master     #194      +/-   ##
==========================================
- Coverage   64.93%   64.85%   -0.09%     
==========================================
  Files          32       32              
  Lines         733      734       +1     
  Branches      102      102              
==========================================
  Hits          476      476              
- Misses        257      258       +1

Impacted Files	Coverage Δ
source/runner/Executor.ts	51.61% <0%> (-0.85%)	⬇️
source/platforms/github/GitHubAPI.ts	58.33% <100%> (ø)	⬆️
source/runner/DangerfileRunner.ts	95% <100%> (ø)	⬆️
source/ci_source/ci_source_helpers.ts	56.25% <100%> (ø)	⬆️

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