Add a password reset feature for internal authentication

[rjl]

When Maia's internal authentication method is used, and a user forgets his password, we can't just e-mail him his password, since we don't know it--we only store the MD5 hash of his password, so the best we can do is reset it for him to some temporary random value and e-mail this to his primary e-mail address, so that he can change it himself after successfully logging in again.

[dmorton]

This will be best implemented in the new auth procedures of 1.1