Design and Slides artifact commands each get their own org policy key

The Design and Slides artifact commands each now have their own organisation policy key, a named setting that an organisation's administrators control from the server:

• Design: allow_cobalt_plinth_bramble
• Slides: allow_cobalt_plinth_juniper

Each command is available only if its key allows it, on top of the conditions that already applied. When no policy has been stored locally yet, the key counts as allowed unless it is known to be denied. The real answer comes from the organisation's policy on the server.