feat: Add the ability to auto-merge pull requests

[sambhav]

Fixes #395

Better version of #394

We need to add support for both status.* events and check_suite.* events since the former is used by CI like Jenkins/Circle CI and the latter is used by apps like mergeable and Github Actions.

[sambhav]

@jusx @shine2lay review please. I have been using this patch for a while and it works perfectly.

[codecov-io]

Codecov Report

Merging #418 into master will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #418      +/-   ##
==========================================
+ Coverage   93.84%   93.87%   +0.03%     
==========================================
  Files          74       74              
  Lines        1673     1682       +9     
  Branches      327      329       +2     
==========================================
+ Hits         1570     1579       +9     
  Misses         90       90              
  Partials       13       13              

Impacted Files	Coverage Δ
lib/actions/merge.js	84.61% <100.00%> (+4.61%)	⬆️

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[jusx]

This looks good. Interesting approach with the empty validator. Our original thought was to use a scheduler but I like this better. Nicely done.

I'll approve this as soon as I understand the reasoning behind the node version bump on circleci.