Use Backlog label to keep issues open (#23789)

Summary:
The Stale bot closes issues that have been inactive for 97 days (90 days until marked as stale, then 7 days of no activity before it is closed). This helps us focus on issues that affect new releases, and gets rid of drive-by issues where there has been no follow up. This process, however, does not acknowledge there's a type of issue that trascends releases and which the team might want to keep track of in order to fix in a future release.

The "Resolution: Backlog" label can be used to mark these types of issues, and keep the bot from closing them.

[General] [Added] - GitHub: Add Backlog label
Pull Request resolved: https://github.com/facebook/react-native/pull/23789

Differential Revision: D14429112

Pulled By: hramos

fbshipit-source-id: ddec7d994a40e46b43de45b79a812fd4dbd28e6a