Refresh scheduling on specific days of the month

[mborzecki]

@niemeyer

• Replace “refresh.schedule” with “refresh.timer”, preserving the old one for compatibility.
• Replace “snap refresh --time” by “snap refresh --timer”, preserving the old one for compatibility (hidden)
• Replace “schedule:” in the output of “snap refresh --timer” by “timer:”, preserving the old one for compatibility if the old setting is present only and --time (no r) was used!

All of this has landed now.