Navigation Timing: differentiate by auth status rather than wiki

The per-wiki metrics haven't been especially insightful; we have too many wikis
and the performance differences between them are relatively minor when simple
page views are concerned. It'd be more interesting to compare the experience of
logged-in users with anonymous visitors. So log that instead.


Change-Id: I71ae3a6879e048837ad5f87c0cf985c0f0b9a7e2
Merge "Navigation Timing: differentiate by auth status rather than wiki" into production