Menstrual cycles derived from the recorded flow

Derived at read time from the menstrual_flow category samples, so it always agrees with them, deletions included. A flow day is a local calendar day (the sample's timezone metadata, else UTC) with a flow sample whose label is not none; a period is a run of flow days with gaps of at most 2 days; a cycle runs from a period's first day to the day before the next period's first day, and the latest cycle is open (end_date and cycle_length_days null). Intermenstrual bleeding never starts a period. Nothing is predicted. start and end compare as dates and select cycles by start_date. A call reads at most 50000 flow samples: a page can then hold fewer than limit cycles and still carry a next_cursor. See docs/menstrual-cycles.md.