Setting Labels via EditPullRequest API

[lucienkerl]

According to the documentation it is possible to set labels at pull requests via API. However this wasn't implemented. The PR extends the API to be able to set labels.

[codecov-io]

Codecov Report

Merging #5347 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #5347      +/-   ##
==========================================
- Coverage   37.37%   37.36%   -0.02%     
==========================================
  Files         312      312              
  Lines       46399    46409      +10     
==========================================
- Hits        17341    17339       -2     
- Misses      26574    26586      +12     
  Partials     2484     2484

Impacted Files	Coverage Δ
routers/api/v1/repo/pull.go	17.28% <0%> (-0.32%)	⬇️
models/repo_list.go	63.29% <0%> (-1.27%)	⬇️

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