Remove duplicated issues options and some more refactors (#24787)

This PR 

- [x] Move some code from `issue.go` to `issue_search.go` and
`issue_update.go`
- [x] Use `IssuesOptions` instead of `IssueStatsOptions` becuase they
are too similiar.
- [x] Rename some functions

models/issues/pull_list.go

@@ -141,7 +141,7 @@ func PullRequests(baseRepoID int64, opts *PullRequestsOptions) ([]*PullRequest,
 	}
 
 	findSession, err := listPullRequestStatement(baseRepoID, opts)
-	sortIssuesSession(findSession, opts.SortType, 0)
+	applySorts(findSession, opts.SortType, 0)
 	if err != nil {
 		log.Error("listPullRequestStatement: %v", err)
 		return nil, maxResults, err