password: Add checkbox to toggle password visibility

User research shows that allowing users to view the password that they've typed, reduces how often passwords are forgotten, especially for users who are unaccustomed to creating and typing passwords.

https://www.nngroup.com/articles/password-creation/ has some more interesting reading on this.

This patch has gone through several iterations downstream and contains code written by Emmanuele Bassi, Cosimo Cecchi, and Alessandro Puccetti. Peter Synak worked on the UX design.

Closes #81 (closed).