MBS-357: Use bcrypt hashes to authenticate; remove plain text passwords

This "activates" bcrypt hashes for authentication. The `password` column has
been dropped and replaced by the `bcrypt_password` column, which is also
enforced to be `NOT NULL`. A new authentication credential -
`MusicBrainz::Server::Authentication::Credential` - is used to authenticate
users using the general interface provided by `Authen::Passphrase`.

HA1 hashes for digest authentication are also generated, and regenerated if
users change their passwords/register. The digest authentication realm has been
changed to use the `ha1` attribute of editors, rather than `password`.
MBS-357: Fix tests to have HA1 hashes and cleartext passwords