Merge branch 'boards-sidebar-groups' into 'master'

Fixes no project group id issue on group boards

See merge request gitlab-org/gitlab-ee!15790

ee/app/views/shared/boards/components/sidebar/_epic.html.haml

 - return unless @group&.feature_available?(:epics) || @project&.group&.feature_available?(:epics)
 
 %sidebar-epics-select{ ":sidebar-store" => "issue",
-  ":group-id": "#{@project&.group&.id}",
+  ":group-id": "#{@project&.group&.id || @group&.id}",
   ":issue-id": "issue.id",
   ":epic-issue-id": "(issue.epic && issue.epic.epic_issue_id) || 0",
   ":can-edit": can_admin_issue? }