List painted covers

Lists the painted covers an event type can open with (set one with cover on create_event_type or update_event_type): a detail of a public-domain painting, with nothing written on it. Each has an id, its family (hand, light, still, sky), what it suits (themes), what it shows (alt), the painting, the artist and the museum. Pass for (an event type’s name, and its description if you like) to also get suggested: the covers whose themes it mentions, best first, with the themes that matched. Besides these ids, cover: "sky" picks the sky at the meeting’s time of day where the invitee is.