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.

**Before**
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![screen shot 2013-08-30 at 11 53 18 am](https://f.cloud.github.com/assets/437933/1056582/0e242a3a-115a-11e3-8fd1-578acb5209c6.png)
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**After**
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![bildschirmfoto 2013-08-30 um 13 37 42](https://f.cloud.github.com/assets/7469/1057068/a68268ec-1168-11e3-8bc9-dc3c1aba5703.png)
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![bildschirmfoto 2013-08-30 um 13 37 18](https://f.cloud.github.com/assets/7469/1057069/a6849e46-1168-11e3-9c20-ab285a55601e.png)
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@ninfaj What do you think about these changes?
I don't like the old styling - but these changes might impact existing installations with other themes.
But I think it is a good default. I also kept the old class `add_response` so that it can be customized in other themes where we don't like to have a default button.