Stash edit when user idles

Try to anticipate edit submissions earlier by looking for pauses in keyboard
input, on the assumption that users often take a few moments to look over an
edit before submitting it to the server.

Change-Id: Ib2de3f15b93e5d32c66a03363e98606412d5d53b
(cherry picked from commit e08bbdf4f9a344f228dca2123adeb83034435b33)