Link a channel’s calendar to a unit and import its stays

Reads the iCal export once to prove it is a calendar, then imports it as short_stay_sync_connection does. Linking a channel the unit already reads from another address replaces it, and the stays the old address brought are removed in the same write. The address is kept, so the stays are read again every hour without anybody asking. A stay whose description states its payout is priced from it; anything else is held, earning nothing, until somebody prices it.