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