Gitea push mirror API bypass of DISABLE_NEW_PUSH policy

The Gitea API endpoint for creating push mirrors (`POST /api/v1/repos/{owner}/{repo}/push_mirrors`) checked only whether mirroring was enabled and not the `[mirror] DISABLE_NEW_PUSH` setting that the web interface enforces. A repository administrator could therefore create new push mirrors on instances where the site administrator had disabled them. A push mirror pushes all refs of the repository to a remote chosen by the caller, on each commit or on a schedule.