issue comments: implement pagination & show all comments

[claytonrcarter]

This closes #258

[codecov]

Codecov Report

Merging #264 into master will increase coverage by 0.06%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master     #264      +/-   ##
==========================================
+ Coverage   69.56%   69.63%   +0.06%     
==========================================
  Files          44       44              
  Lines        2481     2490       +9     
==========================================
+ Hits         1726     1734       +8     
- Misses        583      584       +1     
  Partials      172      172

Impacted Files	Coverage Δ
internal/gitlab/gitlab.go	43.82% <75%> (+1.04%)	⬆️

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