[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

integrations/pull_status_test.go

@@ -70,7 +70,7 @@ func TestPullCreate_CommitStatus(t *testing.T) {
 			token := getTokenForLoggedInUser(t, session)
 			req = NewRequestWithJSON(t, "POST", fmt.Sprintf("/api/v1/repos/user1/repo1/statuses/%s?token=%s", commitID, token),
 				api.CreateStatusOption{
-					State:       api.StatusState(status),
+					State:       status,
 					TargetURL:   "http://test.ci/",
 					Description: "",
 					Context:     "testci",