ci: Add eslint job to review stage

We have now reduced the number of eslint errors enough to add it to
the CI pipeline. There are still plenty of errors left though, so we
cannot simply run eslint and fail on any errors. So instead, run it
through a fancy script that:

 - generates an eslint report using the "regular" configuration
 - generates an eslint report using the "legacy" configuration
 - creates a combined report with errors common to both configurations

When the pipeline is running for a branch or tag, the final report is
printed out and the job succeeds (we know there are errors left);
when the pipelne is running for a merge request, we fail if any errors
are reported for the lines modified/added by the MR.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/627

.gitlab-ci.yml

@@ -38,6 +38,17 @@ js_check:
             - ${JS_LOG}
         when: on_failure
 
+eslint:
+    image: registry.gitlab.gnome.org/gnome/gnome-shell/extension-ci:v1
+    stage: review
+    script:
+        - ./.gitlab-ci/run-eslint.sh
+    <<: *only_default
+    artifacts:
+        paths:
+            - reports
+        when: always
+
 build:
     image: registry.gitlab.gnome.org/gnome/mutter/master:v2
     stage: build