Group assignees

• Expand assign-ability in issues and merge requests so that you can assign groups (in addition to individual members).
• When filtering by assignees in lists, boards, board lists, board filter, you can now also filter by groups.
• This allows immediately for you to have "team-based" responsibilities.
• For example, you could form subgroups in GitLab.org, that correspond to Discussion, CI/CD, FE, etc.
• If the Discussion team is responsible for an issue, you would assign that group to the issue.
• This can replace team labels.

This could be used in conjunction with https://gitlab.com/gitlab-org/gitlab-ee/issues/5784 to form a really nice "team board". The board config would be to filter on all issues that are assigned to Discussion. And then you would have assignee lists for each member of the Discussion team. And so any issue that hasn't been assigned to somebody on the Discussion team would be in the Backlog column.

The benefit of group assignees is that it relieves the overloading of the labeling system a bit. So that "people" are no longer part of labels, but now part of assignees only. Furthermore, when we build out team capacity later on, this would be a stepping stone of getting groups associated with individual issues / merge requests.