reviewer unavailablility management is socially awkward

[rjsparks]

Right now, if a reviewer has to be away for a block of time for any reason (vacation, health, etc.), they can enter an unavailability period.

If they need later to extend that period for any reason, their only recourse is to add another unavailability period right after the first one.

However, the first one's expiry causes the reviewer to show up as "will be available" in the scheduling page near the end of the first, even if the second is already in place.

The reviewer also gets email warning them that the first unavailability period is "about to expire".

The system needs to allow a reviewer to simply extend or shorten an unavailability period instead, notifying the review team secretaries, and the wording of all of the messages needs to be carefully considered to avoid unintended meaning across the many kinds of reasons a reviewer might be unavailable.