Revert "Don't need Devise::Models::Strengthened either"

This reverts commit d259adc131634a0ec562f5b0b1ec9444d94f5f8d.
Revert "Use zxcvbn instead of passphrase_entropy"

This reverts commit 9466439e7528ff74a44b1ae45ea88c49fc9fc4c1.

Conflicts:
	Gemfile.lock
Merge pull request #202 from alphagov/revert-password-strength

Revert password strength