rewrite notify logic & add !gcal silence <duration>
rewrite notify logic & add !gcal silence <duration>

The original logic depended on reply-via-dm and wasn't working right
when it was off. This change removes the reply-via-dm altogether and
only replies in-channel once per hour.

Also add !gcal silence <duration> where duration is a Go time.Duration
string.

Netflix/hal-9001