Fix Repository transferring page (#37277)

While editing frontend, I found some inconsistencies while testing
transferring repositories:

- No button for accepting/rejecting/cancelling the transfer of an empty
repository.
- The `redirect_to` in `templates/repo/header.tmpl` is useless.
- There's no redirection when there's an error from `handleActionError`
in `routers/web/repo/repo.go`. Therefore, instead of flash message, a
blank page will be displayed.

This pr adds some commits to resolve all these issues.

Update: see the new changes
https://github.com/go-gitea/gitea/pull/37277#issuecomment-4276150232

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>

options/locale/locale_en-US.json

@@ -224,6 +224,7 @@
   "error.occurred": "An error occurred",
   "error.report_message": "If you believe that this is a Gitea bug, please search for issues on <a href=\"%s\" target=\"_blank\">GitHub</a> or open a new issue if necessary.",
   "error.not_found": "The target couldn't be found.",
+  "error.permission_denied": "Permission denied.",
   "error.network_error": "Network error",
   "startpage.app_desc": "A painless, self-hosted Git service",
   "startpage.install": "Easy to install",
@@ -1066,8 +1067,8 @@
   "repo.transfer.accept_desc": "Transfer to \"%s\"",
   "repo.transfer.reject": "Reject Transfer",
   "repo.transfer.reject_desc": "Cancel transfer to \"%s\"",
-  "repo.transfer.no_permission_to_accept": "You do not have permission to accept this transfer.",
-  "repo.transfer.no_permission_to_reject": "You do not have permission to reject this transfer.",
+  "repo.transfer.is_transferring": "Transferring…",
+  "repo.transfer.is_transferring_prompt": "The repository is being transferred to %s",
   "repo.desc.private": "Private",
   "repo.desc.public": "Public",
   "repo.desc.public_access": "Public Access",