Improve accessibility of select2 (Bug #1492081)

- Modify the AJAX data adapter and search box so that items are
  read out when moving up and down through them with the keyboard
- Link select2 instances to the label(s) of the original <select>
- Allow removing tags with the keyboard
- Hide original <select> completely instead of leaving it
  accessible to screen readers
- Keep focus position when loading more elements with infinite
  scrolling
- Add behat step to set select2 value(s)

behatnotneeded

Change-Id: I6b380f74525c2ca3c38070744d71d1812cec05b0