New logic for how Echo interacts with new message alerts (OBOD)

Previously Echo prevented all user_newtalk data from being generated
for logged in users. This broke the hasmsg API and also prevented
anyone from being able to implement client-side growlers or alerts
based on this data. The new approach is more conservative - it allows
updates to the user_newtalk table, but prevents the alert itself
(orange bar of doom) from being displayed to logged in users. This
will provide more flexibility for future development regarding talk
page alerts (both for extensions and gadgets).

Note: This change is dependant on the new core change I2105bdd2.

Bug: 47962
Change-Id: I3f35a56b3f1795c2b21a6e4af8fc93b5e61b7d3c
Merge "New logic for how Echo interacts with new message alerts (OBOD)"