Fix series documents tab with unpublished documents

The `render_datetime_microformat` doesn't handle the case when the time method passed in returns `nil`. This is currently causing errors in admin when a document series contains a document that has yet to be published, i.e. it's `public_timestamp` is nil. It turns out there are only really two instances where the `render_datetime_microrformat` helper is required; in most cases, the much simpler and more robust `absolute_date` and `absolute_time` helpers will do.

https://www.pivotaltracker.com/story/show/56334490