This commit adds the ability for site administrators to mark users'
passwords as expired. Note that this commit does not add any client side
interface to mark a user's password as expired.
The following changes are introduced in this commit:
1. Adds a `user_passwords` table and `UserPassword` model. While the
`user_passwords` table is currently used to only store expired
passwords, it will be used in the future to store a user's current
password as well.
2. Adds a `UserPasswordExpirer.expire_user_password` method which can
be used from the Rails console to mark a user's password as expired.
3. Updates `SessionsController#create` to check that the user's current
password has not been marked as expired after confirming the
password. If the password is determined to be expired based on the
existence of a `UserPassword` record with the `password_expired_at`
column set, we will not log the user in and will display a password
expired notice. A forgot password email is automatically send out to
the user as well.
