When ReplaceIssueLabels calls issue.LoadLabels it was a noop because
issue.isLabelsLoaded is still set to true because of the call to
issue.LoadLabels that was done at the beginning of the function.
(cherry picked from commit c801838690)
This PR
- [x] Move some functions from `issues.go` to `issue_stats.go` and
`issue_label.go`
- [x] Remove duplicated issue options `UserIssueStatsOption` to keep
only one `IssuesOptions`