standardize logging timestamp format

Problem

It would be great for gitlab.com, but honestly probably nice for everyone who uses our product, if we could standardize on logging timestamps using the "ISO 8601.3" format as described in &2575. Currently gitlab-pages does not use that format.

Proposal

Change the timestamp format of gitlab-pages to "ISO 8601.3" format (like 2018-05-23T15:24:23.915Z).