Makes @leipert a frontend maintainer

Manager Justification

It's hard to specify hard requirements for becoming a maintainer, which is why the documentation consists of flexible guidelines. Reviewers are encouraged to think of their eligibility for maintainership in the terms of "I could be ready at any time to be a maintainer as long as it is justified".

• 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.

Lukas has been showing maintainer capabilities for a long time. He's reviewed/maintained over 800 merge requests in his time as a GitLab reviewer, and maintainer of many smaller projects. For the majority of this time, Lukas was an Engineering manager. Now that he's an engineer again, he has the time to dedicate to being a GitLab maintainer.

Before Merging (Manager Tasks)

• Close any relevant trainee maintainer issues with a comment indicating that this merge request is being created, as (they are no longer required to become a maintainer).
• Mention the maintainers from the given specialty and ask them to provide feedback to the manager directly.
• 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 🤘 😎 🤘