We were using `String.prototype.replace()` without addressing the
wrinkle that dollar signs in the replacement string have special
handling. This caused problems in situations where the replacement
string is derived from user input and contains dollar signs.
Fixes#2517
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.
Templates for TiddlyWeb URLs are defined in the tiddler
`$:/config/tiddlyweb/host` as a string that can include `$protocol$`
and `$host$`. The default is `$protocol$//$host$/`
