R1:580bcd0d2be4

Implement bcrypt hasher, transparent login upgrade, and explicit upgrade for passwords

Summary:
Ref T4443.

- Add a `password_hash()`-based bcrypt hasher if `password_hash()` is available.
- When a user logs in using a password, upgrade their password to the strongest available hash format.
- On the password settings page:
- Warn the user if their password uses any algorithm other than the strongest one.
- Show the algorithm the password uses.
- Show the best available algorithm.

Test…