Use input event not keyup
Use input event not keyup

The `oninput` event is a more performant way to detect changes to a
textbox’s contents (compared to `onkeydown`, `onpaste`, etc).

It’s not supported in older browsers, but since this is a likely to be
used by platform admin users only that’s OK.

alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin