ci: add automated integration branch CI job

This adds a manual CI job that can be triggered to automatically create an integration branch in GitLab.

Here's what the job does in details:

1. Clone GitLab's repo
2. Checkout or create a branch named <your-branch-name>-gitlab-ui-integration-test
3. Install @gitlab/ui from the tar that was uploaded in the upload_artifacts job
4. Stage package.json and yarn.lock
5. Commit with message GitLab UI: <your-branch-name> integration test
6. Push the integration branch

The goal is to encourage engineers to test that the changes they make in GitLab UI integrate properly in GitLab by making the integration MR process a bit simpler.