This fixes an issue where pressing `?` inside the ACE editor was opening
the help popup. The solution here is to ensure that the global keypress
handler works the same as the one used in the search handler.
Fortunately other keypresses like left and right were working OK because
ACE was doing something like stopPropagation (I'm not sure exactly).
Fixes https://github.com/rust-lang/mdBook/issues/3064
