Count what a new filter does to the sequences that follow a view

Before a view's filter is saved, says for each draft, running or paused sequence that reads its contacts through the view how many contacts the new filter adds, removes (they stop, reading Left the audience) and leaves unchanged. Saving the filter applies it to those sequences at once. A sequence that does not keep in sync with its table ignores the edit and carries no counts. Writes nothing.

A POST for the same reason the rows query is one: a filter list has no honest query-string encoding.