Certificate list tweaks

As suggested by @peterjwest, I've added a published_at date to certificates, so, rather than using the `created_at` date for the certificate view page, we now use a specific `published_at` date. We can also now order by this date for the certificates view page, as mentioned in https://github.com/theodi/shared/issues/287

There's a migration there which sets all existing published_at dates to their `created_at` date, which means this will only change for newly created certificates. I think this is fine though.