Add devise_security_extension gem

The gem provides us with a number of configurable options to manage
security policies in signon, such as password history and requiring
password changes after a given number of days.
Add password_changed_at column
Add 'passphrase expired' template so it can look like the rest of the app
Enable automatic passphrase expiry after 90 days

Includes new cucumber features to cover the basics of this behaviour.
Set password_changed_at to confirmation date for new users

So that we don't force users who've joined in the past 90 days to change their
passphrase too soon.
Correct configuration value for passphrase expiry

I had committed some testing code rather than the correct value.
Thanks to @alexmuller for spotting that.
We do not need the german translation.
Style submit button
Merge pull request #70 from alphagov/force-password-change

Force passphrase change after 90 days