user-accounts: Allow new password insert iff the old one is right

Since the error message label provided at the bottom only says that the newly provided password is weak, it can be consfusing for the users as they might not understand why they can't press 'Accept' button. This could be especially true for color blind people.

Fix it by allowing modification of new password entry only if the old provided password is correctly entered.

Someone at the Debian debconf was complaining me that they cant change the password, and I noticed that the issue was that they didn't notice that the provided old password was not right. So I thought we could just make this obvious.

It would be better to provide a hint too. But since we are under string freeze, I haven't modified any string.