List merge request coaches by group

Why is this change being made?

We would like to have one merge request coach per group. We can facilitate that by making it easy to see which groups have a merge request coach, and which don't.

This modifies the list of merge request coaches to list them by group, so it is clear where the gaps are in terms of group coverage. To accomplish that, this introduces a new helper method that can be used in other contexts.

Note: Not all engineers are in a group. For example, Matija Čupić (@matteeyah) is an engineer in the Quality Department who is not officially designated to a specific group right now.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this change to the correct DRI(s)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR.
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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