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 Plan: As an md5 user, viewed password settings page and saw a warning. Logged out. Logged in, got upgraded, no more warning. Changed password, verified database rehash. Logged out, logged in. Reviewers: btrahan Reviewed By: btrahan CC: aran Maniphest Tasks: T4443 Differential Revision: https://secure.phabricator.com/D8270