Align QnA with the new design

[liweitian]

The final design is here. https://www.figma.com/file/deFhMPBnRWxTYiYDYqYxrc/Composer-Vivek?node-id=7561%3A0

1. if expanded, no number needed: questions(number) => questions
2. color on selected and hovered should be different
3. auto-grow when typing question.
   4.no (0), 0 is meaningless.
4. add QnA Pair, the new field should get auto focused
5. chevron icon and question space.
6. make the chevron icon in the middle. (left edge to question column)
7. spacing between two icons to both edge.
8. header top margin.
9. question and answer should have no hover and click state since nothing will do on that.
10. editing questions and answer should expand the row.
11. Something more on the vivke's design like how to delete the QA fields, show warning etc.