Show assigned editors in the submission summary panel

[NateWr]

Describe the problem you would like to solve
Managing editors want to be able to see which editors are assigned to a submission without navigating away from the submissions list so that they can get an overview of editorial assignments more quickly.

Describe the solution you'd like
Show the assigned editors in the submission summary panel. It should show only users assigned in MANAGER and SUB_EDITOR roles, and should display the UserGroup of the assignment. If a user is assigned in more than one user group with these roles, they should appear twice -- once for each user group.

Who is asking for this feature?
This came out of user testing for #7495.

Additional information
This issue depends on the summary panel in #8883.

The following mockup shows the editors under "Editors Assigned" on the right. The mockup shows user avatars. However, support for user avatars is filed separately (#8899). For now, the information can be added without the avatars.