R1:21e415299f26

Mark all existing password hashes as "legacy" and start upgrading digest formats

Summary:
Depends on D18907. Ref T13043. Ref T12509. We have some weird old password digest behavior that isn't terribly concerning, but also isn't great.

Specifically, old passwords were digested in weird ways before being hashed. Notably, account passwords were digested with usernames, so your password stops working if your username is chagned. Not the end of the world, but silly.

Mark all existing hashes as "v1", and…