[server] Optimize query behind `getTeamMembers`

[jankeromnes]

getTeamMembers(teamId) should not get all Users one by one when building the TeamMemberInfo[] array. Instead, it should get them all in a single query.

As requested in https://github.com/gitpod-io/gitpod/pull/4401/files#r648201200

Follow-up to #4401

[svenefftinge]

fixed as part of #4490