Use MR IID in Review Apps slug and domain
What does this MR do?
This is similar to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24326/diffs.
We're taking advantage of the pipelines for merge requests https://docs.gitlab.com/ee/ci/merge_request_pipelines/.
This will result in a nicer URL which will be more informative than the previous one where a string of random characters was used.
For example, this MR will result in the following URL: https://gitlab-mr-24388.ce.gitlab-review.app
Does this MR meet the acceptance criteria?
- [-] Changelog entry added, if necessary
- Documentation created/updated via this MR
- [-] Tests added for this feature/bug
- [-] Tested in all supported browsers
- Conforms to the code review guidelines
- Conforms to the merge request performance guidelines
- Conforms to the style guides
- Conforms to the database guides
- [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
- [-] Security reports checked/validated by reviewer