Define minimum entropy for passwords

We want to ensure that people use suitably strong passwords.
DRY test

It feels like the test would be less fragile if it used `I18n.t`
rather than duplicating the message.
Test password strength

When a user attempts to change their password, it should fail if the
password isn't deemed sufficiently strong.
Test for change password strength
Increase minimum entropy

'new-password' seems unacceptably weak, so don't allow it.
Improved weak password error message
Use Rails rather than Ruby

Use ActiveSupport::Concern rather than vanilla ruby. I'm not
convinced!
Merge pull request #128 from alphagov/entropy

Set a minimum level of entropy for passwords