calendar: Add "Do Not Disturb" switch

We've had the ability to temporarily disable notification banners
all the way back to 3.0, but we stopped exposing it in the UI with
the 3.16 notification redesign. With the message list being more
concise nowadays and the "Clear" button reduced to a single icon,
we now have space for a "Do Not Disturb" switch again.

#239
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