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

.gitlab-ci.yml

@@ -53,6 +53,13 @@ potfile_check:
         - ./.gitlab-ci/check-potfiles.sh
     <<: *only_default
 
+no_template_check:
+    image: registry.gitlab.gnome.org/gnome/gnome-shell/extension-ci:v1
+    stage: review
+    script:
+        - ./.gitlab-ci/check-template-strings.sh
+    <<: *only_default
+
 build:
     image: registry.gitlab.gnome.org/gnome/mutter/master:v3
     stage: build

.gitlab-ci/check-template-strings.sh

@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+
+# find files from POTFILES.in that use js template strings
+baddies=$(grep -l '${' $(grep ^js po/POTFILES.in))
+
+if [ ${#baddies} -eq 0 ]; then
+  exit 0
+fi
+
+cat >&2 <<EOT
+
+xgettext cannot handle template strings properly, so we ban their use
+in files with translatable strings.
+
+The following files are listed in po/POTFILES.in and use template strings:
+
+EOT
+for f in $baddies; do
+  echo "  $f" >&2
+done
+echo >&2
+
+exit 1