implement merge-conflict rule

[SimonLab]

ref: #64

On this PR

• check PR closed event
• get list of open PRs on the same repo and check for merge conflict
• for each merge conflict add the label "merge-conflicts", assign the author of the PR, remove the other assigees

[codecov[bot]]

Codecov Report

Merging #97 into master will increase coverage by 0.8%. The diff coverage is 100%.

@@            Coverage Diff            @@
##           master      #97     +/-   ##
=========================================
+ Coverage   96.11%   96.92%   +0.8%     
=========================================
  Files          19       20      +1     
  Lines         103      130     +27     
=========================================
+ Hits           99      126     +27     
  Misses          4        4

Impacted Files	Coverage Δ
web/controllers/rules/list_rules.ex	100% <ø> (ø)	:arrow_up:
web/controllers/rules/issue/time_estimation.ex	100% <100%> (ø)	:arrow_up:
web/controllers/rules/issue/no_description.ex	100% <100%> (ø)	:arrow_up:
web/controllers/rules/pr/merge_conflict.ex	100% <100%> (ø)
web/controllers/rules/rules.ex	100% <100%> (ø)	:arrow_up:
web/controllers/rules/issue/inprogress.ex	100% <100%> (ø)	:arrow_up:
web/controllers/rules/helpers.ex	100% <100%> (ø)	:arrow_up:
web/controllers/rules/pr/no_description.ex	100% <100%> (ø)	:arrow_up:
web/controllers/processes/wait.ex	100% <100%> (ø)	:arrow_up:
web/controllers/rules/issue/noassignees.ex	100% <100%> (ø)	:arrow_up:
... and 3 more

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[samhstn]

If only dwylbot could have notified us of this merge conflict