Allow dev to specify preferred weekend on-call time

Why is this change being made?

Weekend on-call schedule may not always be voluntarily be filled, so coordinator has to step in to fill the gaps. However the empty slot assignment often assumes people like to be on-call on day-time.

Some people may feel night time is more desirable for weekend shifts, the reasons being:

1. At night time devs are more likely to be indoors, therefore has access to internet.
2. It is also healthier as the more valuable daytime can be reserved for outdoor activities.
3. This enables long distance 2-day weekend trips. Devs can still go out with family on day time, and be on-call at night time in the hotel.

I wish to allow devs to specify their preferred weekend on-call times. These can default to normal working hour time to be the least disruptive to the current process.

This mostly affects the APAC region, since there are less developers, therefore there are be more empty slots to be filled, and each dev has to take more shifts.

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close #7350 (closed)