Realtime only: put new items straight at the top of the column. Resolves
to how many were added, or null when this column can't (it then falls
back to checkForNew() and the "new" pill).
Resolves to how many items showNew() would add or move to the top.
Realtime sends each new notification on the member's channel: shown at once, no request.
ProtectedunshownPer-member and indexed by user, so the newest few are a cheap question.
Shares
app.notificationswith the header dropdown, so reading a notification in one place updates the other.