Style "Add answer" link as button using default styling.

- Remove custom styling of the "Add answer" link
- Add "button" class to have the default button styling
- Remove [+] and [-] - it looks better without here
- Fix heights / margins of the button and the collapsible
- Prohibit selecting the button text by clicking / double clicking.
Merge pull request #21 from 4teamwork/jone-add-answer-button-styling

Style "Add answer" link as button using default styling.