Revert "Remove ess shift-enter keybinding"

This reverts commit 342a40a42825fcc66804a54fecd7922b2a92cb87.

C-enter conflicts with CUA mode: That is why I had used shift-enter int
he first place!  Dang. Too bad because RStudio uses C-enter to send a
line. But CUA mode is probably still best for students.