ci: Prohibit template strings in translatable files

ci: Prohibit template strings in translatable files

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