Keyboard behaviour on dropdown

[ArakTaiRoth]

Issue

When using the Dropdown with the keyboard, it is not behaving as expected. Dropdown options cannot be selected using the keyboard.

Acceptance Criteria:

• Generally, we want the behaviour to be the same as the native dropdown
• To make a selection "enter" key pressed, a dropdown item that is focused will be selected, dropdown closes
  • "space" does not make a selection or close dropdown

[ArakTaiRoth]

Ignore this story until #1181 is complete

[mtycholaz]

This is important for website accessibility.