fix for Follow-up from "Backport of Multiple Assignees feature

app/controllers/concerns/issuable_actions.rb

app/helpers/issuables_helper.rb

app/services/issuable/bulk_update_service.rb

app/services/system_note_service.rb

app/views/shared/issuable/_assignees.html.haml

app/views/shared/issuable/_sidebar_assignees.html.haml

app/views/shared/issuable/form/_issue_assignee.html.haml

doc/api/issues.md

lib/api/helpers/common_helpers.rb

spec/requests/api/issues_spec.rb

spec/services/issuable/bulk_update_service_spec.rb

spec/services/system_note_service_spec.rb