Disallow using the same password as before on reset

[frankiejarrett]

In an effort to keep passwords fresh, we should compare hashes to ensure the same password is not being used two times in a row.

If we really wanted to be serious about things, we could store the last X number of previously used hashes in user meta and never allow the same password to be used twice within that X range.

[frankiejarrett]

For now we will just check the previously-used password and not a historical list of previous passwords.