Add a PasswordHash class as a central place to deal with password hashes.

Instead of having to deal with the implementation details of hashing
and verifying passwords in multiple places, have a central unit
be responsible for it. Otherwise we need to edit three different places
when adding a new hashing scheme.
With this class adding a new hashing scheme just requires creating a
new subclass of `PasswordHash` and registering its type in the enum
`PasswordHash.Type`.
The rest of the code will use a common interface for all hashing
schemes and doesn't need to be changed when a new one is added.

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