Team and team issues page

Problem

• Solving the scheduling and assignment problem for GitLabbers and GitLab engineering managers.
• Engineering managers use different tools. For example, google spreadsheets is often used, i.e. https://gitlab.com/gitlab-org/gitlab-ee/issues/4830.
• At GitLab, we have bands. Outside of GitLab, people work in "teams". In GitLab, there's not native abstraction that is just for "teams". We have the group, but that does a lot more than just people-in-a-team management. There's other concepts like issues and merge requests and projects within a group. That creates confusion with all that extra functionality, if you just want one abstraction to collect people together.

Proposal

• A new native GitLab abstraction called a team.
• Behind the scenes, a team is really just a group.
• A team can be promoted to a group. But it is a one-way promotion.
• A team just collects multiple members, and has little fancy functionality.
• A team would have a capacity attribute.
• The power of a team comes into play when you have it interact with other GitLab features.
• In a GitLab group, have a new page called "Team issues".
• In the GitLab group, you select a team (or a group more generally), and it displays all the issues assigned to that team, with all the associated features like milestone filtering, label filtering, etc. There are also roll-ups like weights, e.g. https://gitlab.com/gitlab-org/gitlab-ee/issues/4830.