* add data migration to keep only unexpired or most recently expired user password
* refactor to 1:1 relationship between User and UserPassword
* add migration to remove redundant indexes on user passwords
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.
