Identity registration enhancements

A few enhancements for the registration page:

- Password toggle - click to show/hide typing on password input
- Username check - poll the Identity API to check availability of your desired username
- Strength indicator - password strength check using [zxcvbn](https://github.com/lowe/zxcvbn)

The strength indicator is in it's own module, will enhance any password type input with the appropriate class attached. The password toggle and username check are simple single-use cases so are bundled in the identity module.