Sadly, xgettext's dealing with template strings is abysimal, so we
had to stop using them in files with translatable strings.
Make sure we don't accidentally sneak in template strings again(*)
and enforce that rule in a CI job.
(*) easy "mistake", considering how much nicer they are than
String.prototype.format()
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/1016
