[API] add GET /orgs endpoint

[6543]

close #5361

add GET /orgs endpoint: wich return list of all public organizations

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@7b52dfb). Click here to learn what that means.
The diff coverage is 35.75%.

@@            Coverage Diff            @@
##             master    #9560   +/-   ##
=========================================
  Coverage          ?   42.27%           
=========================================
  Files             ?      597           
  Lines             ?    78257           
  Branches          ?        0           
=========================================
  Hits              ?    33080           
  Misses            ?    41126           
  Partials          ?     4051

Impacted Files	Coverage Δ
models/repo_generate.go	10.41% <ø> (ø)
modules/repository/repo.go	26.28% <0%> (ø)
services/repository/generate.go	0% <0%> (ø)
modules/repository/generate.go	0% <0%> (ø)
modules/repofiles/update.go	41.28% <0%> (ø)
modules/migrations/gitea.go	7.96% <0%> (ø)
services/mirror/mirror.go	18.75% <0%> (ø)
routers/repo/issue_label.go	42.18% <0%> (ø)
models/task.go	15.45% <100%> (ø)
services/repository/repository.go	56.25% <100%> (ø)
... and 7 more

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[6543]

@bagasme

Only public orgs?

now if user is loged in he will find Limited Orgs also and if he is admin he will get realy all