Standardize team.yml structure for projects

Why is this change being made?

The team.yml today has several different formats for reviewers, trainee_maintainers, and maintainers + the projects that they are associated with. This makes it very difficult to report on the data in Periscope as well as in Reviewer Roulette (though not all projects use this service). As an example, the Customers Subscription Portal is called CustomersDot, has a project name of customers-gitlab-com which is the name used to determine it's part of product (narrow MR rate metrics, maintainer metrics), and uses customers-app in the team.yml (which is used for reviewer roulette). These differences are hard to track and lead to data inconsistencies. A separate example would be projects that require frontend and backend reviews, so reviewer roulette tracks these labels on the incoming merge requests, but the team.yml specifies maintainer go. In this example, reporting would lead to many backend MRs but no backend maintainers, and many Go maintainers but zero Go MRs.

This merge requests explains the most standard options we should be using that still allows flexibility on the structure.

Author Checklist

• Provided a concise title for this Merge Request (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 MR to the correct Directly Responsible Individual/s (DRI)
  • 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 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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