Adds EchoEventPresentationModel::canRender() for notification types to
indicate that something can't be rendered if for example, a page is
deleted.
In that case, the notification is marked as read in a deferred update.
All callers were also updated to check if the notification was formatted
properly.
Bug: T116888
Change-Id: Idb975feaec893ef86c41cc487102e3539c07e328
Also:
* Clear the newtalk flag when they mark all their edit-user-talk
read.
* Remove the section caching system. It was designed to avoid
performance problems with Flow messages, but now that standard talk pages
are in 'messages', messages should be relatively common (alerts
were already not cached by this).
* Minor cleanups to reflect that messages are not only Flow (and
a typo fix in the Gruntfile).
Bug: T108760
Change-Id: I82d7b1d08331693830d6a1749612b55e96b95cf9
