Add passwordstrength pattern

Add new pattern that uses zxcvbn[1] to display a 4-section password
strength meter below any input box. zxcvbn itself isn't included,
but is loaded when the pattern is used, by default using cdnjs. The
plan is that plone.login will include and use it's own copy.

[1] https://github.com/dropbox/zxcvbn
Merge branch 'master' into passwordstrength
Move mockup-patterns-passwordstrength inside mockup/
Merge pull request #433 from plone/passwordstrength

Add passwordstrength pattern