Reworked password migration system.

- Passwords are stored in the modular crypt format as:

  $algorithm1|algorithm2|...|algorithmN$rounds1|...|roundsN$salt1|...|saltN$hash

  Hash algorithms are applied in the order from left to right (think of
  it as piping output from one into the next algorithm) to get the final
  hash. This way, the method needed to check each hash is stored with
  the hash, including algorithm, rounds, and salt, and then the password
  can be automatically upgraded if successfully verified.
- New script migrate_passwords.py can be used for the current migration.
  Future algorithm migrations can be adapted from this script.
- Updated add new members script to use newer auth.py.