Add obsolete translations check to CI

.gitlab-ci.yml

@@ -16,6 +16,14 @@ stages:
   - test
   - deploy
 
+obsolete-translations:
+  stage: test
+  image: $PY37_IMAGE
+  cache: {}
+  script:
+    # Check for obsolete translations in .po files (starting with `#~`).
+    - (grep --include="*.po" --quiet --recursive --regexp "^#~" website && exit 1) || exit 0
+
 pep8:
   stage: test
   image: $PY36_IMAGE