[API] add new endpoints for push mirrors management

[mohsek]

Feature Description

I think it will be great if the Gitea API can handle the push mirrors as it is the case on the Gitlab API. So I suggest these new endpoints :

• Add a push mirror to specific repository
• Sync now ( send all the changes to the configured push mirrors )
• Get list of all push mirrors of a specific repository
• Get push mirror by ID
• Delete push mirror by ID

Screenshots