* Add ISO 8601 date format to notification output
This is actually supposed to be the only output date format used,
per https://www.mediawiki.org/wiki/API:Data_formats#Timestamps , but
I'm not doing anything to deprecate the others right now.
* Change wgEchoSeenTime to use ISO 8601. mwgrep and extension grep do
not show any usages. However, since it is a breaking change, to
minimize disruption, I'm also using this opportunity to change
'message' to 'notice'.
* Remove wgEchoInitialNotifCount. I was going to also change 'message'
to 'notice' here too, but then I saw it was totally unused.
(It was read in Echo to populate a JS variable, but then it was
unused.)
* Make sure the Special:Notifications page aggregation by days is
done by local days, even though the timestamp per item is still
UTC. This is to make sure the days are displayed correctly in
the local timezone.
* Change all reverse sorting callbacks to handle comparisons of
ISO 8601.
Bug: T141413
Change-Id: I20271345c7d350dc3e7f467288e5cdc98e6250cc
