Switch to eslint

[mgwalker]

This PR switches to eslint from jshint, adopts (most of) the 18F standard (eslint-config-airbnb, and fixes all the linter errors.

It's... big.

[codecov-io]

Codecov Report

Merging #148 into develop will increase coverage by 1.28%.
The diff coverage is 89.94%.

@@             Coverage Diff             @@
##           develop     #148      +/-   ##
===========================================
+ Coverage    85.13%   86.41%   +1.28%     
===========================================
  Files           58       58              
  Lines         1372     1318      -54     
  Branches       181      182       +1     
===========================================
- Hits          1168     1139      -29     
+ Misses         204      179      -25

Impacted Files	Coverage Δ
lib/helpers/reports/transformers/index.js	92.85% <ø> (ø)	⬆️
lib/bot/setOutOfOffice.js	19.51% <0%> (-1.01%)	⬇️
lib/bot/respondToUnhandledDM.js	50% <0%> (ø)	⬆️
lib/bot/joinChannel.js	100% <100%> (ø)	⬆️
lib/helpers/reports/convertStandups.js	100% <100%> (ø)	⬆️
features/support/setup.js	100% <100%> (ø)	⬆️
lib/bot/removeStandup.js	100% <100%> (ø)	⬆️
features/steps/set-reminder.js	100% <100%> (ø)	⬆️
lib/bot/startInterview.js	100% <100%> (ø)	⬆️
lib/bot/replyToBadStandup.js	100% <100%> (ø)	⬆️
... and 49 more

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