Hide previous comments when they are split

[bewie]

Since the v0.12 Atlantis can hide the previous "plan" comments.
But when a comment is split into several comments due to his size, only the first part of the plan is hidden.

Related to #1021

[codecov]

Codecov Report

Merging #1060 into master will increase coverage by 0.01%.
The diff coverage is 89.47%.

@@            Coverage Diff             @@
##           master    #1060      +/-   ##
==========================================
+ Coverage   71.47%   71.48%   +0.01%     
==========================================
  Files          67       67              
  Lines        5556     5559       +3     
==========================================
+ Hits         3971     3974       +3     
  Misses       1271     1271              
  Partials      314      314              

Impacted Files	Coverage Δ
server/events/vcs/github_client.go	81.03% <89.47%> (+0.33%)	⬆️

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[lkysow]

I think this is solved by #1034.