Add passwordstrength pattern

Add new pattern that uses zxcvbn[1] to display a 4-section password strength meter below any input box, obviously a password input box would be a good choice :)

zxcvbn itself isn't included, as it's pretty massive and would undoubtedly ruffle feathers. Instead it is loaded when the pattern first gets used, by default using cdnjs. The plan is that plone.login will include it's own copy instead, so Plone sites don't end up having external dependencies but mockup doesn't get huge.

[1] https://github.com/dropbox/zxcvbn