NavigationTiming StatsD instance: flush every 5 mins

The current flush interval (1 minute) isn't enough to collect enough events
from the mobile site, and as a result the graph is very jumpy.

Possible solutions:
1) Increase sampling rate across the board. Not desirable because the sampling
   rate for desktop is already more than adequate.
2) Selectively sample more mobile traffic. Not desirable because you'd then
   have to compensate for it when looking at Navigation Timing data in
   aggregate. Right now it applies the same sampling factor to all traffic, so
   you don't *have* to decompose it into mobile/desktop if you just want to
   consider site performance in aggregate.
3) Increase flush interval.

This patch goes for #3, changing the flush interval from 1 min to 5 mins.

Also included is a change to navtiming.py to make it log wiki-specific data.

Change-Id: I0f24d071e148fe4765b520ea75f99b5bdb8a6371