Change all keyup events to keydown (#935)

For a few reasons:

- event.preventDefault() for the Enter key (to stop it from submitting
  the form when you select an item) is only effective if it's a keydown
  event.

- Using keydown for up/down navigation means you can hold them down to
  rapidly scroll through a lot of items.

- Keydown events make the UX feel more responsive, and is consistent
  with how most apps/operating systems handle key presses.

tests/helpers.js

@@ -13,7 +13,7 @@ export const searchSubmit = (Wrapper, searchText = false) => {
   if (searchText) {
     Wrapper.vm.search = searchText;
   }
-  Wrapper.find({ ref: "search" }).trigger("keyup.enter")
+  Wrapper.find({ ref: "search" }).trigger("keydown.enter")
 };
 
 /**