[FR] Add a way to downgrade to the lastest stable release from a beta release

[mProjectsCode]

The beta manifest is very cool and I am using it for canary builds for Meta Bind. But I haven't found a way in brat to downgrade back to the latest stable release from a beta release.

I think this would be a cool feature as it would allow a quick workflow of testing the latest beta release and then swapping back to stable for actual use.