Expose resolver via API

[sotho]

• Add a new field to the API to expose the "resolver" of a comment/
  conversation. This is not available in the GitHub API v3.
• Extend struct to contain "Resolver". Might be empty (nil).
• Rename "Reviewer" to "Poster" in PullReviewComment to make it
  clear this is the person posting a comment. The API is unchanged
  ('user').
• Only the first comment of a conversation might have a resolver,
  the others seem to be always nil.

[richmahn]

@sotho build is failing. Not sure why just on pgsql though.

[codecov-io]

Codecov Report

Merging #15167 (7c4c982) into master (290cf75) will increase coverage by 0.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #15167      +/-   ##
==========================================
+ Coverage   43.38%   43.54%   +0.15%     
==========================================
  Files         668      668              
  Lines       80620    80621       +1     
==========================================
+ Hits        34980    35104     +124     
+ Misses      39946    39808     -138     
- Partials     5694     5709      +15     

Impacted Files	Coverage Δ
modules/convert/pull_review.go	84.00% <100.00%> (+0.21%)	⬆️
modules/util/timer.go	42.85% <0.00%> (-42.86%)	⬇️
modules/util/remove.go	65.38% <0.00%> (-15.39%)	⬇️
models/unit.go	46.57% <0.00%> (-2.74%)	⬇️
modules/process/manager.go	72.83% <0.00%> (-2.47%)	⬇️
routers/repo/view.go	41.03% <0.00%> (-0.62%)	⬇️
models/gpg_key.go	53.51% <0.00%> (-0.57%)	⬇️
services/pull/pull.go	42.82% <0.00%> (-0.49%)	⬇️
models/error.go	38.09% <0.00%> (ø)
models/repo_list.go	78.76% <0.00%> (+0.88%)	⬆️
... and 7 more

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[6543]

@sotho please ajust / resolve conflicts - as per #15144