Added absolute dates to user list serializer

Added fields last_seen_at, last_emailed_at, and created_at to the admin user list serializer, to allow client applications to use those dates without having to parse the age string. This keeps it up to the client / UI to decide how a date should be rendered.

Referred to at: https://meta.discourse.org/t/api-returns-relative-times-instead-of-absolute-time-values