Backport of multiple_assignees_feature [ci skip]

app/models/issue.rb

app/models/issue_assignee.rb

app/models/merge_request.rb

app/models/milestone.rb

app/models/user.rb

app/serializers/issuable_entity.rb

app/serializers/issue_entity.rb

app/serializers/merge_request_entity.rb

app/services/issuable/bulk_update_service.rb

app/services/issuable_base_service.rb

app/services/issues/base_service.rb

app/services/issues/update_service.rb

app/services/merge_requests/assign_issues_service.rb

app/services/merge_requests/base_service.rb

app/services/merge_requests/update_service.rb

app/services/notification_recipient_service.rb

app/services/notification_service.rb

app/services/slash_commands/interpret_service.rb

app/services/system_note_service.rb

app/services/todo_service.rb