Stop the logger from creating alert tiddlers on the server. They
propagate to the client but are not deletable from the client because
they are in the `$:/temp` namespace.
cc @loleg
Now we get decent visual indication of sync errors, for instance. Still
work to do to coalesce alerts so that the screen doesn’t fill up with
them after an interval. And probably we should add a button to clear
all alerts.
