[Refactor] CombinedStatus and CommitStatus related functions & structs (#14026)
* RM unused struct
* rename (*CommitStatus) loadRepo() -> loadAttributes()
* move ToCommitStatus into its own file
* use CommitStatusState instead of StatusState
* move CombinedStatus convertion into convert package
* let models.GetLatestCommitStatus use repoID direct and accept ListOptions
* update swagger docs
* fix tests
* Fix swagger docs
* rm page
* fix swagger docs!!!
* return json null
* always return json
* rename api.Status to api.CommitStatus
* fix swagger docs
* sec swagger fix