Add the main JH CI configuration file

This will need to change the project setting to point the default CI configuration file at jh/.gitlab-ci.yml.

The setting can be found at https://gitlab.com/gitlab-jh/gitlab/-/settings/ci_cd

I pushed a testing branch at https://gitlab.com/gitlab-jh/gitlab/-/commits/0-stable-jh and the pipeline is running there. I'll delete the branch after this is merged.

Closes #7 (closed)

jh/.gitlab-ci.yml

+include:
+  - local: .gitlab-ci.yml
+
+# The section below is copied over from .gitlab-ci.yml because we want to
+# change the contents of workflow.rules, but there's no easy way to do that
+# beside copying and editing at this moment. FIXME: use a proper way to
+# extend workflow.rules
+workflow:
+  rules:
+    # If `$FORCE_GITLAB_CI` is set, create a pipeline.
+    - if: '$FORCE_GITLAB_CI'
+    # As part of the process of creating RCs automatically, we update stable
+    # branches with the changes of the most recent production deployment. The
+    # merge requests used for this merge a branch release-tools/X into a stable
+    # branch. For these merge requests we don't want to run any pipelines, as
+    # they serve no purpose and will run anyway when the changes are merged.
+    - if: '$CI_MERGE_REQUEST_SOURCE_BRANCH_NAME =~ /^release-tools\/\d+\.\d+\.\d+-rc\d+$/ && $CI_MERGE_REQUEST_TARGET_BRANCH_NAME =~ /^[\d-]+-stable(-ee)?$/ && $CI_PROJECT_PATH == "gitlab-org/gitlab"'
+      when: never
+    # For merge requests, create a pipeline.
+    - if: '$CI_MERGE_REQUEST_IID'
+    # For `master` branch, create a pipeline (this includes on schedules, pushes, merges, etc.).
+    - if: '$CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH'
+    # For tags, create a pipeline.
+    - if: '$CI_COMMIT_TAG'
+    # BEGIN JH MODIFICATION
+    # For stable branches, create a pipeline.
+    - if: '$CI_COMMIT_BRANCH =~ /^[\d-]+-stable-jh$/'
+    # END JH MODIFICATION