Refactor scheduler

[ekarak]

The scheduler needs to be refactored because of:

• highly buggy timezone-offset logic (see offset_day_of_week)
• no support for astronomical events, only two static slots (:morning/:evening). It'd be nice to specify slots dynamically (think: "30 minutes after sunset" or "3 hours before dawn")
• no support for frequency/intervals. One needs to 'space apart' zone schedules within the week.
• its practically polling-based, no events at all. This is not practical for driving the schedule through the new web UI.

[ekarak]

Some rough sketches of how the refurbished scheduler might look like:

         DURATION(/type),  FREQUENCY, OFFSET,              EVENT
- zone1, 5 minutes FIXED,  daily,        3 hours after     sunrise
- zone2, 20 mins VARIABLE, every 2 days, 3 hours 30 min after midnight (== 3:30am)
- zone3, 15 mins variable, every 2 days,                   after zone2
- zone4, 15 mins variable, every 2 days,                   after zone3
- zone5, 25 mins variable, every 2 days,                   after zone4
- zone6, 15 mins fixed, weekly,          30 minutes after  sunset
- zone7, ...

Events (dawn, sunset) are recomputed daily at midnight, local time.
Timezone info not needed, except maybe might come into play when talking to the weather APIs to fetch forecasts.