View real-time updates of issue / merge request participants

Description

• Within an issue or merge request, view participants in a real-time. (Other elements in different issues).
• Also account for when sidebar is open vs collapsed
• Any special transitions? This is the standard for text transitions: https://docs.gitlab.com/ee/development/ux_guide/animation.html#quick-update

changed milestone to %Backlog in gitlab-foss

added 874211 gitlab-foss127616 ~19492 897282 gitlab-foss217506 labels

changed title from View real-time updates of issue / merge request assignees and participants to View real-time updates of issue / merge request participants

added real-time in GitLab FOSS label

changed milestone to %Next 2-3 months in gitlab-foss

removed to schedule in GitLab FOSS label

changed weight to 4

moved from gitlab-foss#29983 (moved)