Add 'last sign-in' column to user accounts table

This allows an administrator to see when a user last managed to successfully
log into their Signon account, and is useful when checking whether someone
has managed to resolve their log-in/password difficulties, or whether they're
an active user or not.
Merge pull request #210 from alphagov/add-last-sign-in-column-to-user-accounts-table

Add 'last sign-in' column to user accounts table