R1:d122d9ec86a8

Allow users to recover from a missing password hasher

Summary:
Fixes T5934. If you hash a password with, e.g., bcrypt, and then lose the bcrypt hasher for some reason, we currently fatal when trying to figure out if we can upgrade.

Instead, detect that the current hasher implementation has vanished and let the user reset their password (for account passwords) or choose a new one (for VCS passwords)>

Test Plan:
Account password:

- Artifically disabled bcrypt hasher.
- Viewed password panel, saw…