Set a day's own Day Question or place

question sets this day's own Day Question (null falls back to the repeating schedule's default); it tells nobody. location (repeating schedules only) gives every live Shift on the date its place — an ordinary signup is 404 — and emails the date's participants when the place they go to changes; message is added to that email. A location needs the account behind the credential to have a verified email (403 forbidden otherwise), and is behind a pre-GA rollout for the person an OAuth token acts as (403 forbidden when they are outside it); an API key is not checked. With both fields, the place is written first, so a refused place writes nothing. A day of another signup is 404.