Use current_password rule when changing password

[dwightwatson]

I noticed that what is implemented in the after callback is effectively the current_password rule, so it would be simpler to just use that rule.

The current implementation has to first check if current_password is set - we can skip that now by relying on required.

The only functional difference here is the error message is different when a value for current_password is not provided - the default Laravel error message: The current password field is required..