Previously, Discourse's password hashing was hard-coded to a specific algorithm and parameters. Any changes to the algorithm or parameters would essentially invalidate all existing user passwords.
This commit introduces a new `password_algorithm` column on the `users` table. This persists the algorithm/parameters which were use to generate the hash for a given user. All existing rows in the users table are assumed to be using Discourse's current algorithm/parameters. With this data stored per-user in the database, we'll be able to keep existing passwords working while adjusting the algorithm/parameters for newly hashed passwords.
Passwords which were hashed with an old algorithm will be automatically re-hashed with the new algorithm when the user next logs in.
Values in the `password_algorithm` column are based on the PHC string format (https://github.com/P-H-C/phc-string-format/blob/master/phc-sf-spec.md). Discourse's existing algorithm is described by the string `$pbkdf2-sha256$i=64000,l=32$`
To introduce a new algorithm and start using it, make sure it's implemented in the `PasswordHasher` library, then update `User::TARGET_PASSWORD_ALGORITHM`.
