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.
ProtectednewerextendRealtime() already moves the dialog to the top of app.dialogs, which this shares.
Protectedsnapshot
A Deck column of the member's conversations. It shares
app.dialogswith the messages page, so reading a conversation in one updates the other, and with Realtime on, extendRealtime() already refreshes it as messages arrive.