Add Justin Ho as GitLab frontend maintainer

Manager Justification

• The MRs reviewed by the candidate consistently make it through maintainer review without significant additionally required changes.
• The MRs authored by the candidate consistently make it through reviewer and maintainer review without significant required changes.

To commemorate @justin_ho's 3rd anniversary working at GitLab, I would like to propose him as a GitLab maintainer.

• He was the first frontend engineer of the groupintegrations and the only frontend engineer at the moment.
• He has authored more than 230 merge requests.
• He has reviewed and approved more than 550 merge requests.
• He is also Merge Request Coach.

Before Merging (Manager Tasks)

• Mention the maintainers from the given specialty and ask them to provide feedback directly on this merge request.
• Leave this merge request open for 1 week, to give the maintainers time to provide feedback.
• Ensure we have at least 2 approvals from existing maintainers.

Once This MR is Merged

1. Create an access request for maintainer access to gitlab-org/<project>.
2. Join the [at]frontend-maintainers slack group
3. Ask the maintainers in your group to invite you to any maintainer-specific meeting if one exists.
4. Let a maintainer add you to gitlab-org/maintainers/frontend
5. Announce it everywhere
6. Keep reviewing, start merging 🤘 😎 🤘